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Practice Guideline 11: A Detailed Examination of the Data on Surgical Abortion and Preterm Birth

October 1, 2022
Edition: Fall 2022
Volume: 37
Issue: 2
Article: 6

Table of Contents

Abstract

Overwhelming evidence from 168 studies over fifty years points to a clear dose-response relationship between surgical abortion and subsequent preterm birth. The 2018 National Academy of Sciences report considered only five of these 168 studies and represents a biased sample that underreports a significant association between surgical abortion and subsequent preterm birth. The purpose of this document is to review the quality of the data on this effect, review the size of this effect, and portray an accurate assessment of the data to improve informed consent prior to surgical abortion.

Figures

AAPLOG-v37i2-PG11-Table-1.-Rubric-for-Evaluating-the-Scientific-Merit-of-Studies-on-Abortion-History-and-Subsequent-Preterm-Birth
AAPLOG-v37i2-PG11-Table-2.-Application-of-Criteria-to-Published-Studies-from-2004-to-2018-Preterm-Birth-37-weeks
AAPLOG-v37i2-PG11-Table-3.-Application-of-Criteria-to-Published-Studies-from-1980-to-2018-for-Very-Preterm-Birth-28-32-weeks

Background

Preterm Birth

An overview of preterm birth (PTB) and its relationship with abortion is provided separately (see Practice Guideline 5). However, the incidence of PTB is important to establish for the statistics presented in this deeper review. PTB is defined as delivery before term, i.e. before 37 weeks and affects about one in ten deliveries in the United States. The majority (70%) of babies born before 37 weeks are born at 34 to 36 weeks. About 10% of PTB (1-2% of all U.S. deliveries) occur before 32 weeks and are termed “very preterm births.” Very preterm births pose greater risks to the neonate and greater costs to the family and system.

For this reason, some studies analyze deliveries before 37 weeks and deliver before 32 weeks (or even lower gestational ages) separately in order to give nuanced meaning to their results. In this document, very preterm birth will be specified as delivery before 32 or 28 weeks, and when PTB and these deliveries are discussed in quick succession, PTB may be spelled out specifically as delivery before 37 weeks.

The NAS Report

The National Academy of Sciences (NAS) recently released a report on the safety of abortion.1 This report addressed the purported association between induced abortion and PTB, but limited the studies they used to assess this link. Their criteria for studies included:

  • Objective documentation of prior abortion (excluding spontaneous abortion, i.e. miscarriage)
  • Comparison of women with prior abortion (the study group) with women with no abortion history (a control group)
  • Statistical methods that control for mental health prior to the abortion (if mental health is an outcome)
  • Published in 2000 or later, including abortions performed in 1980 or later (studying current abortion methods)
  • Similar clinical settings and care delivery to the United States

The authors further stated that the studies meriting attention and discussion should control for confounding variables, such as smoking status, maternal age at abortion, type of abortion (surgical or medication), weeks of gestation at abortion, and number of previous abortions.

The authors posited that, of 168 studies linking PTB to surgical abortion, only five met their criteria for inclusion. Even if the criteria set forth are appropriate, there over 70 studies that meet these criteria (see Appendix A). However, no explanation is provided for omitting such a large portion of the medical literature. While the report did admit that multiple abortions increase the risk for PTB, their conclusions about overall safety misrepresent the data.

The majority of the data on this topic is on surgical abortion, and that is the focus of this document is the association between PTB and surgical abortion, even though some medication abortion outcomes are included in the studies discussed. Here, for simplicity’s sake, surgical abortion for termination of pregnancy is referred to as “abortion.” Miscarriage and medication abortion will be specifically described as spontaneous abortion (SAB) and medication abortion respectively. “Induced abortion” is a term that appears in the literature on this topic because there is often mixing of outcomes between elective and spontaneous abortion. However, this document will simply use “abortion,” and contrast it with SAB.

Woolner et al. (2014) is the major study that the NAS relies on to conclude there is no association between abortion and PTB in a subsequent pregnancy.2 Woolner et al. 2014 includes data from a single site in Scotland from 1986 to 2010. However, this paper’s conclusion contradicts the findings of other studies by two of its own coauthors.

One of these studies (Battacharya et al. 2012) uses the same Scottish database examined by Woolner et al., but find an increased risk of preterm birth (PTB) among women after surgical abortion, compared to women with no abortion, with a relative risk (RR) of 1.37 (95% CI 1.32-1.42). 3 This increase in risk is statistically significant, meaning it is unlikely due to chance, as can be seen from the 95% confidence interval that does not cross 1.0 (1.0 represents no change from the baseline risk). The 95% confidence interval means we can be 95% sure that the true result falls between 1.32 and 1.42, and if it included 1.0, we could not be sure that abortion had any effect on PTB. This specific RR means that women with a prior abortion are 37% more likely to experience a subsequent PTB, increasing their rate from 10% to about 14%.

Battacharya et al. had several strengths over Woolner et al. First, it included a larger number of women (457,477 women without a prior abortion and 120,033 with a history of abortion). Second, Bhattacharya et al. 2012 adjusted their analysis for smoking, but Woolner et al. was unable to adjust for this known confounder in PTB studies. Third, Bhattarya et al. also controlled for the type of abortion performed (medication or surgical). In contrast, Woolner et al. included failed medication abortions that required subsequent surgical completion with the total surgical abortion numbers. Fourth, Bhattarya et al. utilized known gestational age (i.e. < 13 weeks) to evaluate for risk of PTB on a national level, not a single site as had Woolner et al.3 For these reasons, Woolner et al. is a poorer study to rely upon, given that a similar but larger dataset exists and contradicts the smaller, less well-designed study.

Early Evidence of an Association

Papers that examined multiple smaller studies (reviews) on abortion and PTB first emerged in the United States in 2003.10,11 Rooney and Calhoun (2003) reviewed studies from 1966-2003 and found 49 studies with a statistically significant risk for PTB after abortion.11

Meanwhile, the association between abortion and PTB has been known in the international community since at least 1973.21 The Hungarian government was warned about the evidence of a link between abortion and PTB thanks to work by Dr. Jeno Sarkany.12 As a result, Hungary passed restrictive legislation regarding elective abortion, citing increased social and medical burden from PTB. This legislation reduced the abortion rate in Hungary from 57% of all pregnancies in 1969 to 38% in 2000.13

Evidence in the Early 21st Century

The meta-analysis by Swingle et al. (2009) was performed authors who held different political beliefs on abortion, to reduce bias.16 This team reviewed 7,891 titles, 349 abstracts, and 130 manuscripts, finally identifying 12 papers about the risk of PTB after abortion and 9  papers on PTB after spontaneous abortion (SAB) with data available for analysis.

Four of the 12 studies on abortion had data available for common odds ratios (OR) to calculate the odds of PTB less than 32 weeks associated with surgical abortion.

The common OR for these studies was 1.64 (95% CI 1.38-1.91).16 Odds ratios are different from relative risk, but this result is equivalent to a change in the rate of delivery before 32 weeks from about 1.5% (the U.S. baseline rate before 32 weeks), to about 2.3% after one abortion.

This study also found an increased risk of PTB after SAB. Out of the 9 studies available to pool a common odds ratio for PTB after SAB, 7 had data for use in calculations. The authors found that the odds of PTB less than 37 weeks after one SAB was 1.43 (95% CI 1.05-1.66), and with more than 2 SABs, 2.27 (95% CI 1.98-2.81).16

Of note, PTB after abortions is not related to PTB after SAB.  The causes of SAB are internal to the woman or embryo, and may also predispose the mother to preterm birth, especially after recurrent SAB. However, this is different from the cause of abortion, which is a mechanical dilation and removal of the fetus despite the mother’s capacity to carry him. Further, abortion is an avoidable epidemiological risk factor for PTB; SAB, on the other hand, is an unfortunate, often unpreventable, outcome of a desired pregnancy for most women.

Shah et al. conducted a separate analysis in the same year as Swingle et al. (2009).17 These authors screened 834 papers and identified 22 studies on PTB after abortion, which included 268,379 women.17

Shah et al. found a significantly increased risk for PTB after one abortion (OR 1.36, 95% CI 1.24-1.50).17 These odds mean the rate of birth before 37 weeks after one abortion is 13%, compared to the baseline 10%. Seven of these 22 studies reported rates of PTB after two or more abortions, including 158,421 patients. Among these women, there was an increased risk for PTB (OR 1.93, 1.38-2.71).17 This translates to an increase in risk from 10% to about 18%, nearly doubling the risk. These ORs and related increases in rate of PTB to between 13% and 18% demonstrate a dose effect of abortion: the more abortions, the higher the subsequent risk of PTB.

Oppenraaij et al. (also 2009) combined 13 studies and found increased risk of very PTB (birth before 32 weeks) as well as PTB before 37 weeks with one abortion. They also detected a dose effect with more than 2 abortions.18 The authors conclude

a history of TOP [termination of pregnancy] is associated with an increased risk for PPROM, PTD, and VPTD.  These risks depend on the number of TOP.18 

Lowit et al. (2010) also found an increased risk of PTB before 37 and 32 weeks in an analysis that combined 7 systematic reviews (including 4 meta-analyses), one prospective study, 12 retrospective studies, and five case-control studies.19 The authors conclude that “[c]urrent evidence … suggest an association between IA [induced abortion] and pre-term birth.”19

More Recent Evidence

Saccone et al. (2016) included 36 studies in a systematic review and meta-analysis; 31 of these looked at abortion, and 5 looked at dilation and curettage (D&C) after SAB. A total of 1,047,683 women were included among all these studies.20 The authors controlled for bias with best practices including planning analysesbefore selecting included studies, having two authors select studies, using the Methodological Index for Non-Randomized studies, and performing the Higgins test for heterogeneity across studies. Women with one prior abortion had a significantly increased risk of PTB (OR 1.52, 95% CI 1.08-2.16), translating to a risk increase from 10% to 14%.20 The authors concluded that “prior surgical evacuation of the uterus may be an independent risk factor for PTB.”20

In 2020, Laelago et al. performed a systematic review and meta-analysis of abortion and PTB in East Africa.  Their study included 58 studies with 134,801 participants. Pooled analysis of four studies found that prior abortion or stillbirth was significantly associated with PTB. The adjusted odds ratio of PTB in this study was 3.93 (95% CI 2.70-5.60), which is dramatically different from other ORs on this topic. This may be a result of the mixing of stillbirth (and possible SAB) and abortion, which are different physiological entities and result in different management. This is a weakness of this study. The strength of this study consists of the inclusion of eleven East African countries finding similar increased PTB risks with abortion.22 While this study needs confirmation, it suggests that affects from abortion on PTB may span across ethnicities and geographic regions.

Another Approach to Preterm Birth

Since the NAS report is missing significant parts of the available body of data, another attempt at listing and assessing the quality of studies is provided in this document. A rubric was utilized to evaluate the quality of the studies linking abortion history with PTB (see Table 1). This rubric included nine criteria: sample size, generalizability, consent to participate rate, abortion concealment, control for potentially confounding variables, inclusion of a control group, strength of measures or preterm birth, prospective data collection, and attrition rate (longitudinal studies only).  Each criterion was worth 0-4 points for a total of 36 points.

Studies on surgical abortion and delivery before 37 weeks are laid out in Table 2, and studies on very preterm birth are laid out in Table 3. A few are worth describing in more detail. Freak-Poli, et al. (2009) used data from South Australia from 1998-2003 and included maternal smoking history. This  study encompassed 42,269 deliveries with 39,191 term births and 3,078 PTBs.23 They also demonstrated a dose effect: after one abortion, the adjusted odds ratio (aOR) for PTB was 1.35 (95% CI 1.08-1.68), and after two or more abortions, this jumped to 1.63 (95% CI 1.28-2.08).23 These odds ratios translate to an increase in risk from the baseline 10% to Voigt, et al. (2009) evaluated 8 German federal states in a retrospective cohort study of 247,593 women delivering their first child preterm.24 The rate of PTB for women with one prior abortion was 7.8% and for more than 2 abortions, 8.5%. In contrast, only 6.5% of the control group, who had no prior abortion, delivered preterm, a statistically significant difference (p = 0.015).24 A weakness of this study is that the data on prior abortion was self-reported, and some patients may have concealed this. However, concealment tends to weaken associations, because the women concealing their history distribute any effect of abortion into the control group, making the groups behave more uniformly. Thus, concealment in this case might be hiding an even larger effect of PTB. The evaluation of the quality of this study was 29 out of a possible 36 points.

Ancel et al. (2004) is a case control study of 2,938 PTBs and 4,781 controls at term from 10 European countries. This study found increased odds of preterm birth before 28 weeks after one abortion (OR 1.34, 95% CI 1.08-1.68), and even higher odds of delivery before 28 weeks with two or more abortions (OR 1.82, 95% CI 1.34-2.49).25 These odds ratios are similar to those from other studies, but the corresponding elevation in risk of PTB will vary based on the baseline rate of PTB in each included country; in the U.S., these translate to a PTB risk of about 13% after one abortion, and about 15% with two or more abortions, which is consistent with other studies described earlier. One of the key strengths of the study was the internal validation of the database with patient records regarding. demographics, previous pregnancy outcomes, gestational age, hypertension,  IUGR, and antepartum hemorrhage (see Table 2).23 The evaluation of the quality of this study was 21 out of a possible 36 points.

There were 3 informative studies on PTB (before 37 weeks) and abortion in 2011.23,26,27 The Di Renzo et al. database-linked study was a multicenter cross-sectional evaluation of preterm vaginal delivery in 9 centers in Italy.27 The authors eliminated cesarean deliveries from their analysis due to the inability to control for the varying trends in indication for these deliveries. The records were linked to outcomes at each center within the central database.  The investigators performed a power analysis prior to beginning the research. They determined that 6,000 women would be necessary in their population to see a statistically significant difference in the PTB rate in their population. Their sample included 7,634 vaginal deliveries. The authors performed a multivariable regression to assess confounding variables, but did not differentiate between number of prior abortions or types of obstetric history (e.g. did all prior pregnancies end in abortion, or was there one abortion after prior full-term deliveries).

Di Renzo et al. found an increased odds of PTB of (OR 1.954, 95% CI 1.1623.285), which corresponds to an increase from their baseline PTB of 5% to about 9%. The evaluation of the quality of this study was 33 out of a possible 36 points.

The evaluation of the quality of Bhattacharya et al. (2012) previously discussed, was 27 out of 36 points.

Finally, Malosso et al. (2018) studied the rate of PTB compared to abortion between 2003 to 2012 in U.S. databases (which are not linked).38 Specifically, this study used data from National Vital Statistics Reports and Center of Disease and Prevention. This study found the progression toward more medication abortion and fewer surgical abortions was significantly associated with the decrease in PTB in the U.S. since 2001 (p < 0.05).38 The study suffered from lack of linkage of the data and correlation coefficients as a quantitative assessment.  The correlation coefficient only assesses the co-variation as opposed to causation.  Also, the authors did not address the magnitude of the secular trend to decrease iatrogenic preterm births during the study period. This could bring bias into the data collected as a result of changes in general practice not related to induced abortion. The evaluation of the quality of this study was 22 out of a possible 36 points.

A comprehensive list of studies on surgical abortion and preterm birth is provided in Appendix A.

Another Approach to Very Preterm Birth

Just as delivery before 37 weeks needed a comprehensive approach, so too does very preterm birth, or delivery before 32 weeks (in some studies, 28 weeks). Very preterm birth only represents about 1-2% of PTB in the U.S. but results in significant cost and morbidity due to infant prematurity. The same rubric was utilized to evaluate studies on very preterm birth (see Table 3).

Levin et al. (1980) compared pregnancy loss and PTB before 28 weeks with those who delivered at term (after 37 weeks).28 Women who had two or more induced abortions had a 2- to 3-fold risk of very preterm birth. The evaluation of the quality of this study was 25 out of a possible 36 points.

Lumley (1998) provided the RR of very preterm birth of a woman’s first singleton according to her prior obstetric history (no prior pregnancy, prior abortion, or prior miscarriage).29 The paper includes 243,679 deliveries between 1983 to 1992 in Australia. Women who had an abortion had a higher risk of delivery before 28 weeks and before 32 weeks compared to women with no prior pregnancy. This demonstrated a dose effect.29 Weaknesses of the study included possible confounding with regard to maternal age, marital status, birth defect, tobacco, socioeconomic status, and alcohol use.  In spite of this, the author notes:

The data meet four of the criteria for causality.  The temporal sequence is clear: the abortions preceded the preterm birth.  The association is a strong one.  There is a dose-response relationship: the greater the number of prior pregnancies the higher the relative risk.  The association is plausible: possible mechanisms exist.29

The evaluation of the quality of this study was 33 out of a possible 36 points.

Moreau et al. used data from the EPIPAGE study, which evaluated delivery between 22 and 32 weeks in nine French regions.30 The study included 1,943 deliveries before 33 weeks, 276 deliveries between 33 and 34 weeks, and 618 unmatched term controls (39-40 weeks). After abortion, women had increased odds of delivery between 22 and 27 weeks (OR 1.8, 95% CI 1.1-2.8) and between 28 and 32 weeks (OR 1.7, 95% CI 1.0-2.8). The study’s strength was its control for confounding variables.  The evaluation of the quality of this study was 28 out of a possible 36 points.

Smith et al. (2006) analyzed risk with induced abortion and spontaneous PTB in 84,391 first births in Scotland between 1992 and 2001.31 A strength of this study is the use of Cox proportional hazards modeling to determine the association between abortion and the increase in risk of PTB. The authors found an increased risk of PTB at 24-32 weeks with a hazard rate of 1.19 (95% CI 1.06–1.34) with one abortion and a 1.9 (95% CI 1.44–2.49) with two or more abortions, demonstrating a dose effect with a positive trend test (p < 0.001).31 The evaluation of the quality of this study was 33 out of a possible 36 points.

Klemetti et al. (2012) compared 300,858 women experiencing their first delivery between 1996 and 2008 and used the Finnish abortion registry between 1983 and 2008 to understand which women had undergone abortions prior to this delivery.32 31,083 women had one abortion before their first continued pregnancy, 4513 had two abortions, and 93 had three or more abortions. Women with one prior abortion had nonsignificantly increased odds of delivery before 28 weeks (aOR 1.19, 95% CI 0.981.44), but this became significant after 2 abortions (aOR 1.69, 95% CI 1.14-2.51) and for more than 3 abortions (aOR 2.78, 95% CI 1.48-5.24).32 The study’s strength was its completeness of records (excludes recall bias or concealment), and their exhaustive adjustment for confounders.  The evaluation of the quality of this study was 34 out of a possible 36 points.

Scholten et al. (2013) investigated PTB after abortion using national registry study from the Netherlands.33 In 16,000 women with a prior abortion, there were increased odds of delivery before 32 weeks (aOR 1.52, 95% CI 1.26-1.85) and before 28 weeks (aOR 1.67, 95% CI 1.30-2.15). A weakness of the study was its use of self-report of abortions, rather than registry data. The authors concluded that

[w]omen who have had a termination of pregnancy have an increased risk of preterm delivery, cervical incompetence treated by cerclage, placental problems, and PPH [postpartum hemorrhage]

The evaluation of the strength of the quality of the study was 27 out of a possible 36 points.

Hardy et al. (2013) used a Canadian database (the McGill Obstetric and Neonatal Database) to examine deliveries before 26, 28, and 32 weeks after a prior abortion.34 The study included 17,916 women between 2001 and 2006, of whom 2,276 (13%) had undergone one prior abortion, and 862 had undergone two or more abortions. The study described increased adjusted odds of delivery before 32 weeks (aOR 1.45, 95% CI 1.11-1.90), before 28 weeks (aOR 1.71, 95% CI 1.21-2.42), and before 26 weeks (aOR 2.17, 95% CI 1.41-3.35).34 A limitation of the study was self-report to disclose a history of induced abortion.  However, self-reporting tends to favor the null hypothesis if women do not disclose abortion. This would sort themselves incorrectly into the control group, equalizing the effects in both groups. A second limitation was the failure to differentiate whether the abortions were medication or surgical abortion, and whether they were done in the first or second trimester. The evaluation of the quality of this study was 25 out of a possible 36 points.

Zhou et al. (2014) performed a population–based prospective study of preterm prelabor rupture of membranes (PPROM) in 14 cities in China from 2001 to 2012.35 112,439 women were included in the analysis, of whom 3,077 (2.7%) had PPROM. Women were at increased odds of PPROM before 28 weeks after abortion (OR 2.75, 95% CI 1.66-4.56). The strength of the study is the ability to control for smoking, alcohol, medical history comorbidities, a family history of medical diseases, history of spontaneous miscarriage, fetal death, and fetal anomalies. The evaluation of the quality of this study was 34 out of a possible 36 points.

Usynina et al. (2016) using registry data from all 52,806 live births in a Russian county from 2006 to 2011.36 Women who had undergone surgical abortion were at increased odds for delivery before 28 weeks (aOR 1.96, 95% CI 1.32-2.91) and delivery between 28 and 32 weeks (aOR 1.36, 95% CI 1.06-1.76). The strengths of this study were the ability to control for the morbidities of educational level, marital status, alcohol abuse, and diabetes and the large size. Limitations include possible under-reporting of alcohol abuse, pre-pregnancy BMI, and the lack of separation of induced and spontaneous miscarriages. The evaluation of the quality of this study was 32 out of a possible 36 points.

Situ et al. (2017) reported on 419,879 first deliveries with a singleton between 1996 and 2003.37 Women who had a prior abortion had increased odds of delivering before 28 weeks (OR 1.51, 95% CI 1.03-2.23). Strengths of the study include the large number of first-time mothers with singleton births over an 18-year time frame, use of national registry linked data, and ability to analyze for induced abortions in multiple categories.  Limitations of the study include lack of data on interpregnancy intervals and socioeconomic status.  The authors attempted use smoking as a proxy for socioeconomic status. The evaluation of the quality of this study was 34 out of a possible 36 points.

A comprehensive list of studies on abortion and very preterm birth is provided in Appendix B.

Clinical Questions and Answers

Q What About the Increased Risk of PTB Due to D&C Alone, Regardless  of Abortion?

Lemmers et al. (2016) confirmed the association between PTB and D&C.  This meta-analysis reviewed 21 studies, including a total of 1,853,017 women who had undergone D&C for abortion or SAB.21 Compared to women with no history of D&C, women with a prior D&C for any reason had an adjusted odds ratio of 1.29 for PTB (95% CI 1.17-1.42), and an adjusted odds ratio of 1.69 for PTB before 32 weeks (95% CI 1.20-2.38). This translates to an increased rate of birth before 37 weeks of 13% (from 10%) or birth before 32 weeks of 2.5% (from 1.5%). These results for very preterm birth are consistent with 31 other studies demonstrating a significantly increased risk of PTB with surgical abortion and D&C in general. (See Appendix B.)

Women with a history of multiple D&Cs compared with those with no D&C had an OR of 1.74 for PTB (95% CI 1.10-2.76), meaning an increase from 10% to 16%.

Lemmers concluded, “D&C is associated with an increased risk of subsequent preterm birth.  The increased risk in association with multiple D&Cs indicates a causal relationship.  Despite the fact that confounding cannot be excluded, these data warrant caution in the use of D&C for miscarriage and termination of pregnancy, the more so since less invasive options are available.”21

This conclusion also concurs with Malosso et al., which finds that the rate of PTB has declined as medication abortions replace some surgical abortions.38

Rather than allowing us to dismiss the association between surgical abortion and PTB as “just due to D&C,” this data confirms that the very procedure we are using to end pregnancy is the cause of increased risk of PTB. We, as women’s healthcare professionals, must critically hold ourselves and our profession accountable for counseling women about risks related to the procedure or intervention.

Q What About the Increased Risk of PTB Due to Short Interval Pregnancy  after Abortion?

Short interval pregnancy, or short interpregnancy interval, is defined as a new pregnancy less than six months after the end of the prior pregnancy. The NAS report investigated whether the increased risk of PTB after abortion is due to short interval pregnancy. That report concluded that the association between PTB and short interval pregnancy is inconsistent and may be related to other factors found in other studies.6

A recent examination of short interpregnancy interval using a better statistical model (within-mother analysis vs. between mother analysis) is thought to better assess confounding risk factors, like abortion. When within-mother analysis is used, the risk of PTB attributed to short interpregnancy interval alone is not significant (OR 1.07, 95% CI 0.86-1.34). This means that the higher ORs seen for abortion and PTB cannot be due to short interpregnancy interval alone.7 The same result was shown with the use of conditional logistic regression, another technique meant to assess for confounding factors: short interpregnancy interval was not associated with PTB in 38,178 Canadian deliveries.8

Interestingly, the interval between pregnancies tends to be longer after abortions as shown in a 2017 analysis of 173,205 U.S. birth certificates. The same study showed that the number of previous abortions was not correlated with interpregnancy interval.9

Q Observational Studies Cannot Prove Causality by Definition, so How Can the Association between Abortion and PTB Ever be Proven as Causal?

Prospective controlled studies cannot be done on autonomy-related behaviors such as abortion or tobacco use, since this would be unethically coercive.

The authors of some studies on abortion and PTB openly assert that their study cannot aid in proving causality because they are observational,32 but the same assertion may be made regarding tobacco’s association with lung cancer. Clinicians must act on the statistically sound observational data to establish reasonable certitude in clinical practice with regard to causation and guide their recommendations accordingly.

Q Does the Increased Rate of PTB after Abortion Concur with Low Birth  Weight Outcomes?

Low birth weight (LBW) is defined as birth weight less than 2500 grams and occurs in 8% of deliveries in the United States. Out of the 18 studies on LBW analyzed by Shah et al. (2009), there were 280,529 patients available to compare at the level of individual patient data. The authors compared women with no abortions prior to their first delivery to women with one abortion prior to their first delivery. There was a significantly increased risk for LBW after one abortion (OR 1.35, 95% CI 1.20-1.52).17 This means that from a baseline rate of 8%, the rate of LBW rises to about 11% after one abortion. Only 5 of 18 studies included LBW findings after two or more abortions, representing 49,347 patients. Using these patients, the pooled OR for LBW after two or more abortions was 1.72 (95% CI 1.45-2.04), meaning an increased rate from 8% to 13%. This difference in the rate of LBW after one (11%) and two or more (13%) abortions shows a dose effect: the more abortions a woman undergoes prior to her first delivery, the higher the risk that her first neonate will have LBW.17

Saccone et al. (2010) also looked at LBW, and found an OR of 1.41 (95% CI 1.22-1.62) after one abortion. While Shah et al. did not find a statistically significant increase in small for gestational age (SGA) infants after abortion, Saccone et al. found a significant increase, with an odds ratio of 1.19 (95% CI 1.01-1.42).

Q Most of the Above Data is about First Trimester Surgical Abortion. What is the Evidence for Second Trimester Abortion and Preterm Birth?

The NAS authors used the study by Woolner et al. 20142 and the study by Jackson et al. 20074 to evaluate the risk of PTB following medication and surgical abortion done later than 13 weeks. Both studies are unable to state whether later abortions are associated to an increase risk for PTB, but the NAS report does not include this disclaimer. Mirmilstein et al. 2009, a small study of 77 women who underwent second-trimester abortion with misoprostol, did find that this type of second trimester abortion was an independent risk factor for PTB.5

Q What is the Cost of Abortion-Related Prematurity?

A 2007 analysis reviewed studies done through 2005 on this topic, finding 59 studies that demonstrated an increased rate of PTB after abortion and translated the costs of abortion-related prematurity to $1.2 billion annually.14, 23 Ten years later, McCaffrey (2017) estimated there had been a total of $52-57 billion in abortion-related hospital costs due to very preterm birth between 1973 and 2016.15 These calculations did not include any of the costs after discharge related to the morbidity of prematurity, including cerebral palsy, retinopathy, bronchopulmonary dysplasia, deafness, and early intervention programs. As of December 2021, no one has yet to dispute these estimates of the impact on healthcare dollars by abortion.

Q Have Authors on this Subject Minimized Their Positive Findings?

Oppenrajj et al. (2009) attempts to attribute the increased rate of PTB after surgical abortion to confounders (smoking, unemployment, socioeconomic status, short interpregnancy interval), but later admit that there is an association.18

Lowit et al. (2010) write that the “effects of IA [induced abortion] on subsequent reproduction is sparse and conflicting” despite their review of 7 systematic reviews (including 4 meta-analyses), one prospective study, 12 retrospective studies, and five case-control studies, and their own conclusion that abortion is associated with PTB.19

Liao et al., (2011) buried an important clinical and statistical findings in their paper about medication abortions. Medication abortion before 7 week that requires D&C for completion was associated with increased odds of preterm birth (OR 1.69, 95% CI 1.02-3.16) and very preterm birth (OR 3.61, 95% CI 1.43-4.93). Combined, these outcomes occurred in 1 out of 10 patients who needed D&C after medication abortion, but this finding did not make it into the abstract.

Finally, the NAS report itself ignores the substantial body of literature regarding induced abortion and its association with PTB. About 77 studies meet their stated criteria, but are ignored in their analysis, while other studies (e.g. Woolson et al 2014) are included, despite not fulfilling these criteria perfectly.

Q If the NAS Report Admits that Abortion is getting Safer, Shouldn’t We Expect to see Some Increased Risk of PTB in Past Studies?

Yes, an increased rate of PTB in past studies and a disappearance of this effect in more recent studies would be consistent with an improvement in the technique of abortion, making it less risky to women’s future reproductive health.

There are a very few old studies (e.g. Levin et al. 1980) which demonstrate a very high increase in the rate of PTB after surgical abortion, but these are outliers. The majority of the meta-analyses and individual studies from the 1970s through the 2020s have demonstrated a significant, consistent increase in the risk of PTB after surgical abortion, regardless of the purported modernity of the method.  

Summary of Recommendations and Conclusion

The following Recommendations are Based on Good and Consistent Scientific  Evidence (Level A):

  1. The report on abortion safety by the National Academy of Sciences does not reflect the majority of the literature on the increased risk of preterm birth after abortion.
  2. One prior surgical abortion is associated with a statistically significantly higher odds of subsequent preterm birth (PTB), corresponding to a 13-14% risk, compared to the baseline rate of 10% in the United States.
  3. Surgical abortions are associated with a “dose effect,” meaning an increased number of abortions confer increasing risk of PTB.
  4. Two or more prior surgical abortions is associated with significantly higher odds of subsequent preterm birth, corresponding to a 18% risk of subsequent preterm birth, compared to the baseline rate of 10% in the United States. 
  5. One prior surgical abortion is associated with significantly higher odds of having a subsequent very preterm birth (either 32 or 28 weeks’ gestation), corresponding to a 2.3% risk, compared to the baseline rate of 1.5% in the United States.
  6. One prior surgical abortion is associated with significantly higher odds of low birth weight (LBW), corresponding to an 11% risk of subsequent LBW compared to the baseline rate of 8% in the United States.
  7. Two or more prior surgical abortions is associated with is associated with significantly higher odds of low birth weight (LBW), corresponding to a 13% risk of subsequent LBW compared to the baseline rate of 8% in the United States.
  8. The odds and corresponding risk of delivery before 37 weeks and before 32 weeks after D&C for any reason, are similar to the respective rates of delivery before 37 weeks and before 32 weeks after surgical abortion: 13% for one procedure, and 16% for multiple procedures.

The following Recommendations are Based on Limited and Inconsistent Scientific Evidence (Level B):

  1. The etiologies of subsequent preterm birth after surgical abortion, compared to miscarriage or stillbirth, are different and should be approached differently.
  2. Abortion-related prematurity has cost the United States more than $50 billion dollars since Roe v. Wade.
  3. The increased rate of preterm birth after surgical abortion is likely related to the surgical procedure itself.

The following Recommendations are Based Primarily on Consensus and Expert Opinion (Level C):

  1.  The increased risk of preterm birth after surgical abortion should be  included in informed consent for surgical abortion.

Appendix A: Studies on Surgical Abortion and Preterm Birth

XDenotes studies that included miscarriages and stillbirths as well as surgical abortions but did not report separate PTB/LBW risks
^^Denotes studies that found dose/response (the more abortions, the higher the risk)

  1. Laelago T, Yohannes T, Tsigo G. Determinants of preterm birth among mothers who gave birth in East Africa: systematic review and meta-analysis. Italian J of Pediatrics 2020;46:10-23. https://doi.org/10.1186/s13052-020-0772-1.
  2. Giang HTN, Pozza SBD, Tran HT, Ulrich S. Stillbirth and preterm birth associated factors in one of the largest cities in central Vietnam. Acta Paediatrica 2019;108:630-636.
  3. Magro Malosso ER, Saccone G, Simonetti B, Squillante M, Berghella V. US trends in abortion and
    preterm birth. J Matern Fetal Neonatal Med. 2018 Sep;31(18):2463-2467. doi: 10.1080/14767058.2017.1344963. Epub 2017 Jul 6. PMID: 28629238.
  4. Kelkay B, Omer A, Teferi Y, Moges Y. Factors associated with singleton preterm birth in Shire Shul General Hospital, Northern Ethiopia, 2018. J of Pregnancy 2019; https://doi.org/10.1155/2019/4629101.
  5. Soltani M, et al. Assessing the risk factors before pregnancy of preterm births in Iran: a population-based case-control study. BMC Pregnancy and Childbirth 2019;19:57. https://doi.org/10.1186/s12884-019-2183-0.
  6. Zafran N, Musa M, Zuarrez-Easton S, Garmi G, Roman S, Salim R. Risk of preterm birth and low
    birthweight following consecutive surgical and medical abortions. Arch Gynecol Obstet 2017;296:763-769. Doi 10.1007/s00404-017-4474-x.
  7. Saccone G, Perriera L, Berhella V. Prior uterine evacuation of pregnancy as independent risk factor for preterm birth: as systematic review and metaanalysis. AJOG 2016 http://dx.doi.org/10.1016/j.ajog.2105.12.044.
  8. Situ KC, Gissler M, Virtanen SM, Klemetti R. Risk of Adverse Perinatal Outcomes after Repeat Terminations of Pregnancy by their Methods: a Nationwide Register-based cohort study. Paediatric
    Perinatal Epidemiology August 2017. http://onlinelibrary.wiley.com/doi/10.1111/ppe.12389/full
  9. Lemmers M, Verschoor MAC, Hooker AB, Opmeer BC, Limpens J, Huirne JAF, Ankum WM, Mol
    BWM. Dilation and curettage increases the risk of subsequent preterm birth: a systematic review
    and meta-analysis. Human Reproduction 2016;31(1):34-45. Doi10.1093/humrep/dev274.^^
  10. Giri A, Sedhain R. Socioeconomic and Reproductive Factors Related to Low Birth Weight Babies.
    J Nobel Medical College 2016; Volume 5 (Number 1, Issue 8):57-60. X
  11. Roozbeh N, Moradi S, Soltani S, Zolfizaden F, Hasani MT, et al. Factors assocaited with preterm labor in Hormozgan province in 2013. Electronic Physician September 2016;8(9):2918-2923. ncbi.nlm.nih.gov/pmc/articles/PMC5074750/pdf/epj-08-2918.pdf. X
  12. Oster RT, Toth EL. Longitudinal Rates and Risk Factors for Adverse Birth Weight Among First Nations Pregnancies in Alberta. Journal Obstetrics Gynaecology Canada 2016;38(1):29-34. http://www.jogc.com/article/S1701-2163(15)00012-2/pdf. X
  13. Usynina AA, Postoes VA, Grjiborski AM, Krettek A, Nieboer E,et al. Maternal Risk Factors for Preterm Birth in Murmansk County, Russia: A Register-Based Study. [IN PRESS] Paediatric Perinatal Epidemiology May 2016. doi/10.1111/ppe.12304.
  14. Tehranian N, Ranjbar M, Shobeiri F. The Prevalence Rate and Risk Factors for Preterm Delivery in Tehran, Iran. J Midwifery Reproductive Health 2016;4(2):600-604. jmrh.mums.ac.ir/article_6605_71c61dc2599d72332127986bcf4d8b9a.pdf. X
  15. Feresu SA, Harlow SD, Woelk GB. Risk Factors for Low Birthweight in Zimbabwean Women: A Secondary Data Analysis. Plos one June 26, 2015. journals.plos.org/plosone/article/file?id=10.1371/journal. pone.0129705&type=printable.
  16. Patel PK, Pitre DS, Bhooker S. Predictive Value of Various Risk Factors For Preterm Labor. National J Community Medicine 2015;6(1):121-125. njcmindia.org/uploads/6-1_121-125.pdf
  17. Abelhady AS, Abdelwahid A. Rate and Risk Factors of Preterm Birth in a Secondary Health Care Facility in Cairo. World Journal Medical Sciences 2015;12(1):09-16. idosi.org/wjms/12%281%2915/2. pdf X
  18. Passini R, Cecatti JG, Lajos GJ, Tedesco RP, Nomura ML, Dias TZ, Haddad SM, RREhdeer PM, Pacagnella RC, Costa ML, Sousa MH. Brazilian multicentre study on preterm bireth (EMIP): Prevalence and factors associated with spontaneous preterm birth. PLOS ONE 2014;9(10) e109069. Doi10.1371/ journal.pone0109069.
  19. Bugssa G, Dimtsu B, Alemayehu M. Socio Demographic and Maternal Determinants of Low Birth Weight at Mekelle Hospital, Northern Ethiopia: A Cross Sectional Study. American Journal Advanced Drug Delivery 2014X
  20. Makhlouf MA, Clifton RG, Roberts JM, Myatt L, et al. Adverse Pregnancy Outcomes among Women with Prior Spontaneous or Induced Abortions. American J Perinatology 2014.
  21. Chaman R, Amiri M, Ajami M-E, Sadeghian A, Khosravi A. Low Birth Weight and Its Related Risk Factors in Northeast Iran. Iran J Pediatrics December 2013;23(6):701-704. ncbi.nlm.nih.gov/pmc/ articles/PMC4025130/pdf/IJPD-23-701.pdf. X
  22. Haghighi L, Najmi Z, et al. Twin’s sex and risk of pre-term birth. J Obstetrics Gynaecology Nov. 2013;33:823-826.
  23. McCarthy F, Khashan AS, North RA, Rahma M, Walker JJ, et al. Pregnancy loss managed by cervical dilation and curettage increases the risk of spontaneous preterm birth. Human Reproduction Advanced Access 19 September 2013 pp. 1-10. http://www.ncbi.nlm.nih.gov/pubmed/24052504.
  24. Oliver-Williams C, Fleming M, Monteath K., Wood AM, Smith GCS. Changes in Association between Previous Therapeutic Abortion and Preterm Birth in Scotland, 1980 to 2008: A Historical Chort Study. PLOS July 2013;10(7:1-11).
  25. Scholten BL, Page-Christiaens CML, Franx A, Hukkelhoven CWPM, Koster MPH. The influence of pregnancy termination on the outcome of subsequent pregnancies: a retrospective cohort study. BMJ OPEN 2013;3;e002803 doi:10.1136/bmjopen-2013-002803. bmjopen.bmj.com/content/3/5/e002803. full.pdf+html
  26. Raisanen S, Gissler M, Saari J, Kramer M, Heinon S. Contribution of Risk Factors to Extremely, Very and Moderately Preterm Term Births- Register-Based Analysis of 1,390,742 Singleton Births. PLOS ONE April 2013;8(4):1-7 http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0060660
  27. Watson LF, Rayner J-A, Forster D. Identifying risk factors for very preterm birth: A reference for clinicians. Midwifery May 2013; 29(5):434-439.
  28. Ghislain Hardy, Alice Benjamin, Haim A. Abenhaim. Effects of Induced Abortions on Early Preterm Births and Adverse Perinatal Outcomes. Journal Obstetrics Gynaecology Canada 2013;35(2):138 143. http://www.jogc.com/article/S1701-2163(15)31018-5/pdf
  29. Heaman M, Kingston D, Chalmers B, Sauve R, Lee L, Young D. Risk Factors for Preterm Birth and Small-for-gestational-age among Canadian Women. Paediatric Perinatal Epidemiology 2013;27:54-61.X
  30. Klemetti R, Gissler M, Niinimaki M, Hemminki E. Birth outcomes after induced abortion: a nationwide register-based study of first births in Finland. Human Reproduction 2012 August 29 ncbi.nlm.nih.gov/pubmed/22933527
  31. Bhattacharya S, Lowit A, Bhattacharya S, Raja EA, Lee AM, Mahmood T, Templeton A. Reproductive outcomes following induced abortion; a national register-based cohort study in Scotland. BMJ OPEN 2012;2:e000911. doi:10.1136/bmjopen-2012-000911. http://bmjopen.bmj.com/content/2/4/e000911. full.pdf ]
  32. Deshpande JD, Phalke DB, Bengal VB, Peeyuusa D, Bhatt S. Maternal Risk Factors For Low Birth Weight: A Hospital Based Case-Control Study in Area of Western Maharashtra. National J Community Medicine 2011;2(3):394-398.X
  33. Renzo GCD, Giardina I, Rosati A, Clerici G, Torricelli M, et al. Maternal risk factors for preterm birth: a country based population analysis. European J Obstetrics Gynecology and Reproductive Biology. December 2011;159(2):342-346. sciencedirect.com/science/article/pii/S0301211511005392X
  34. Liao H, Wei Q, Duan L, Ge J, Zhou Y, Zeng W. Repeated medical abortions and the risk of preterm birth in subsequent pregnancies. Arch Gynecol Obstet 2011;289:579-586. springerlink.com/content/ d5mt821806512570/
  35. Furquim MA, Alencar GP, Schoeps D, Novaes HMD, Campbell O, et al. Survival and risk factors nor neonatal mortality in a cohort of very low birth weight infants in the southern region of San Paulo city, Brazil. Cadernos de Saude Publica 2011;27(6).
  36. unli G, Weiyue Z. Influence of artificial abortion on preterm labor and risk in subsequent pregnancy. Chinese J Obstet Gynaecol Pediatrics 2010;447-451.
  37. Ammar M Alfadhli, Ali M Hajia, Farida AK Mohammed, Hamdiya A Alfadhli, Medhat K El-Shazly. Incidence and Potential Risk Factors of Low Birth Weight Among Full Term Deliveries 2010;46(2):157- 164.
  38. Watson LF, Rayner J-A, King J, Jolley D, Forster D, Lumley J. Modelling sequence of prior pregnan- cies on subsequent risk of very preterm birth. Paediatric and Perinatal Epidemiology 2010;24:416-423. ^^
  39. Watson LF, Rayner J-A, King J, Jolley D, Forster D, Lumley J. Modelling prior reproductive history to improve predication of risk for very preterm birth. Paediatric Perinatal Epidemiology 2010;24:402- 415. ^^
  40. Yuan W, Duffner AM, Chen L, Hunt LP, Sellers SM, Bernal AL. Analysis of preterm deliveries below 35 weeks’ gestation in a tertiary hospital in the UK. A case-control survey. BMC Research Notes 2010;3:119.

2000-2009

  1. Voigt M, Henrich W, Zygmunt M, Friese K, Straube S, Briese V. Is induced abortion a risk factor in subsequent pregnancy? Journal Perinatal Medicine 2009;37:144-149.
  2. Freak-Poli R, Chan A, Gaeme J, Street J. Previous abortion and risk of preterm birth: a population study. J Maternal-Fetal Med Jan. 2009;22(1):1-7.
  3. Hldre K, Rahu K, Karro H, Rahu M. Previous history of surgically induced abortion and complications of the third stage of labour in subsequent normal vaginal deliveries. J Maternal-Fetal Neonatal Medicine 2008;21(12):884-888.
  4. Visintine J, Berghella V, Henning D, Baxter J. Cervical length for prediction of preterm birth in women with multiple prior induced abortions. Ultrasound Obstetrics Gynecology 2008;31(2):198-200.
  5. Sareer Badshah, Linda Mason, Kenneth McKelvie, Roger Payne, Paulo JG Lisboa. Risk factors for low birthweight in the public hospitals at Peshawar NWFP-Pakistan. BMC Public Health 2008; 8:197-206.X
  6. Voigt M, Olbertz D, Fusch C, Krafczyk D. Briese V, Schneider KT. The influence of previous pregnancy terminations, miscarriages, and still-birth on the incidence of babies with low birth weight and premature births as well as somatic classification of newborns. Z Geburtshilfe Neonatol 2008;212:5-12.^^
  7. Reime B, Schuecking BA, Wenzlaff P. Reproductive Outcomes in Adolescents Who Had a Previous Birth or an Induced Abortion Compared to Adolescents’ First Pregnancies. BMC Pregnancy and Childbirth 2008;8:4.
  8. Brown TS, Adera T, Masho SW. Previous abortion and the risk of low birth weight and preterm births. J Epidemiol Commun Health 2008;62:16-22.X
  9. Curry AE, Vogel I, Drews C, Schendel D, Skogstrand K, et al. Mid-pregnancy maternal plasma levels of interleukin 2, 6, and 12, tumor necrosis factor-alpha, interferon-gamma, and granulocyte-macrophage colony-stimulating factor and spontaneous preterm delivery. Acta Obstectica et Gynecologica 2007:86:1103-1110.
  10. Chung-Chin Lo, Jenn-Jeih Hsu, Ching-Chang Hsieh, T’sang-T-sang Hsieh, Tai-Ho Hung. Risk Factors For Spontaneous Preterm Delivery Before 34 Weeks of Gestation Among Taiwanese Women. Taiwan J Obstet Gynecol 2007;46(4):389-394.
  11. Jackson JE, Grobman WA, Haney E, Casele H. Mid-trimester dilation and evacuation with laminaria does not increase the risk for severe subsequent pregnancy complications. Intl J Gynecol Obstet 2007;96:12-15
  12. Briunsma F, Lumley J, Tan J, Quinn M. Precancerous changes in the cervix and risk of subsequent preterm birth. BJOG Jan. 2007;114(1):70-80.
  13. Losa SM, Gonzalez E, Gonzalez G. Risk Factors for Preterm Birth. Prog Obstet Ginecol 2006;49(2):57-65
  14. Teramoto S, Soeda A, Hayashi Y, Urashima M. Physical and socioeconomic predictors of birth weight in Japan. Pediatrics International 2006;48(3):274-277.
  15. Smith GCS, Shah I, White IR, Pell JP, Crossley JA, Dobbie R. Maternal and biochemical predictors of spontaneous preterm birth among nulliparous women: a systematic analysis in relation to degree of prematurity. Intl J Epidem 2006;35(5):1169-1177.
  16. K.K. Roy, Jinee Baruah, Sunesh Kumar, Neena Malhotra, A.K. Deorari, J.B. Sharma. Maternal Antenatal Profile and Immediate Neonatal Outcome in VLBW and ELBW Babies. Indian Journal of Pediatrics 2006;73(8):669-673
  17. Samin A, Al-Dabbagh, Wafa Y Al-Taee. Risk factors for preterm birth in Iraq: a case-control study. Pregnancy and Childbirth. BMC 2006;6:13.
  18. Poikkens P. Unkila-Kallio L, Vilska S, Repokari L. et al. Impact of Infertility Characteristics and treatment modalities on singleton pregnancies after assisted reproduction. Reproductive Biomed July 2006;13(1):135-144
  19. Etuk SJ, Etuk IS, Oyo-Ita AE. Factors Influencing the Incidence of Pre-term Birth in Calabar, Nigeria. Nigerian J Physiological Sciences 2005;20(1-2):63-68. ajol.info/index.php/njps/article/ viewFile/32656/63772.
  20. Stang P, Hammond AO, Bauman P. Induced Abortion Increases the Risk of Very Preterm Delivery; Results from a Large Perinatal Database. Fertility Sterility Sept 2005;S159.
  21. Conde-Agudelo A, Belizan JM, Breman R, Brockman SC, Rosas-Bermudez. Effect of the interpregnancy interval after an abortion on maternal and perinatal health in Latin America. Int J Gynaecol & Obstet 2005;89 (Supp. 1):S34-S40.X
  22. Moreau C, Kaminski M, Ancel PY, Bouyer J, et al. Previous induced abortions and the risk of very preterm delivery: results of the EPIPAGE study. British J Obstetrics Gynaecology 2005;112(4):430-437. ^^
  23. Burguet A, Kamiski M, Abraham L, Schaaf J-P, Cambonie G, et al. -The complex relationship between smoking in pregnancy and very preterm delivery. Results of the Epipage study. BJOG 2004;111: 258-265. onlinelibrary.wiley.com/doi/10.1046/j.1471-0528.2003.00037.x/pdf.X
  24. Umeora OUJ, Ande ABA, Onuh SO, Okubor PO, Mbazor JO. Incidence and risk factors for preterm delivery in a tertiary health institution in Nigeria. J Obstetrics Gynaecology Nov. 2004; 24(8):895-896.
  25. Ancel PY, Lelong N, Papiernik E, Saurel-Cubizolles MJ, Kaminski M. History of induced abortion as a risk factor for preterm birth in European countries: results of EUROPOP survey. Human Repro 2004; 19(3): 734-740.^^
  26. Reime B, Schuecking BA, Wenzlaff P. Perinatal outcomes of teenage pregnancies according to gravidity and obstetric history. Annals of Epidemiology 2004;14(8):619-619.
  27. El-Bastawissi AY, Sorensen TK, Akafomo CK, Frederick IO, Xiao R, Williams MA. History of Fetal Loss and Other Adverse Pregnancy Outcomes in Relation to Subsequent Risk of Preterm Delivery. Maternal Child Health J 2003;7(1):53-58.
  28. Han WH, Chen LM, Li CY. Incidences of and Predictors for Preterm Births and Low Birth Weight Infants in Taiwan. Chinese Electronic Periodical Services 2003:131-141.
  29. Balaka B, Boeta S, Aghere AD, Boko K, Kessie K, Assimadi K. Risk factors associated with prematurity at the University of Lme, Togo. Bull Soc Pathol Exot Nov 2002;95(4):280-283.
  30. Grimmer I, Buhrer C, Dudenhausen JW. Preconceptional factors associated with very low birth weight delivery: a case control study. BMC Public Health 2002; 2:10.
  31. Henriet L, Kaminski M. Impact of induced abortions on subsequent pregnancy outcome: the 1995 French national perinatal survey. BJOG 2001;108(10):1036-1042. onlinelibrary.wiley.com/doi/10.1111/ j.1471-0528.2001.00243.x/full.
  32. Letamo G, Majelantle RG. Factors Influencing Low Birth Weight and Prematurity in Botswana. J Biosoc Sci 2001;33(3): 391-403.
  33. Ancel PY, Saurel-Cubizolles M-J, Renzo GCD, Papiernik E, Breast G. Risk factors for 14-21 week abortions: a case-control study in Europe. Human Reproduction 2000;15(11):2426-2432.
  34. Bettiol H, Rona RJ, Chin S, Goldani M, Barberi M. Risk Factors Associated with preterm births in Southeast Brazil: a comparison of two birth cohorts born 15 years apart. Paediatric Perinatal Epidemiol 2000;14(1):30-38.
  35. Gardosi J, Francis A. Early Pregnancy predictors of preterm birth: the role of a prolonged menstruation-conception interval. BJOG 2000;107(2):228-237X
  36. Foix-L’Helias L, Ancel, Blondel B. Risk factors for prematurity in France and comparisons betweeen spontaneous prematurity and induced labor; results from the National Perinatal Survey 1995. J Gy necol Obstet Bio Reprod (Paris) Feb 2000;29(1);55-65.
  37. Foix-L’Helias L, Ancel PY, Blondel B. Changes in risk factors of preterm delivery in France between 1981 and 1995. Paediatric and Perinatal Epidemiology. Oct 2000;14(4): 314-323.

1990-1999

  1. Ancel PY, Saurel-Cubizolles, Di Renzo GC, Papiernik E, Breart G. Social Differences of very preterm birth in Europe: interaction with obstetric history. American J Epi 1999;149(10):908-915.X
  2. Zhou W, Sorenson HT, Olsen J. Induced Abortion and Subsequent Pregnancy Duration. Obstetrics & Gynecology 1999;94: 948-953.X
  3. Ancel PY, Saurel-Cubizolles M-J, Renzo GCD, Papiernik E,  Breart G. Very and moderate preterm births: are the risk factors  different? British J Obstetrics and Gynaecology 1999;106:1162-1170. X^^
  4. Lee KS, Lee WC, Meng KH, Lee Ch, Kim SP. Maternal Factors Associated with the Premature Rupture of Membrane in the Low Birth Weight Infant Deliveries. Korean J Prev Med 1998;21(2): 207-216.
  5. Small Babies in Scotland A Ten Year Overview 1987-1996. Information and Statistics Division. The National Health Service in Scotland. Scottish Program for Clinical Effectiveness. Edinburgh 1998 ISBN 1-902076-07-9.
  6. Martius JA, Steck T, Oehler MK, Wulf K-H. Risk factors associated with preterm (<37+0 weeks) and early preterm (<32+0 weeks): univariate and multi-variate analysis of 106 345 singleton births from 1994 statewide perinatal survey of Bavaria. European J Obstetrics & Gynecology  Reproductive Biology 1998;80:183-189. ^^
  7. Lumley J. The association between prior spontaneous abortion, prior induced abortion and preterm birth in first singleton births. Prenat Neonat Med 1998;3:21-24. ^^
  8. Jacobsen G, Schei B, Bakketeig LS. Prepregnant reproductive risk and subsequent birth outcome among Scandinavian parous women. Norsk Epidemiol 1997;7(1):33-39.X
  9. Chie-Pein Chen, Kuo-Gon Wang, Yuh-Cheng Yang, Lai-Chu See. Risk factors for preterm birth in an upper middle class Chinese population. Eur J Obstet Gynecol Reprod Bio 1996;70(1):53-59.
  10. Hagan R, Benninger H, Chiffings D. Evans S, French H. Very preterm birth – a regional study. Part 1: Maternal and obstetric factors. BJOG 1996;103:230-238.X
  11. Lang JM, Lieberman E, Cohen A. A Comparison of Risk Factors for Preterm Labor and Term Small-for-Gestational-Age Birth. Epidemiology 1996;7:369-376.^^
  12. Meis PJ, Michielutte R, Peters TJ, Wells HB. Factors associated with preterm birth in Cardiff, Wales. Amer J Obstet Gynecol 1995; 173:590-596.
  13. Khalil AK, El-Amrawy SM, Ibrahim AG, et al. Pattern of  growth and development of premature children at the age of two and three years in Alexandria, Egypt. Eastern Mediterranean Health  Journal 1995;1(2):186-193.
  14. Hillier SL, Nugent RP, Eschenbach DA, Krohn MA,et al. Association Between Bacterial Vaginosis And Preterm Delivery Of A Low-Birth-Weight Infant. NEJM 1995;333: 1737-1742.X
  15. Guinn D, Goldenberg RL, Hauth JC,  Andrews WA et al. Risk factors for the development of preterm premature rupture of membranes after arrest of preterm labor. AJOG 1995;173 (4):1310-1315.
  16. Herceg A, Simpson JM, Thompson JF. Risk factors and outcomes associated with low birthweight delivery in the Australian Capital  Territory 1989-90. J Paediatrics Child Health Aug 1994;30(4):331-335.
  17. Lekea-Karanika V, Tzoumaka-Bangoula C. Past obstetric history of  the mother and its association with low birth weight of a subsequent  child: a population-based study. Paediatr Perinat Epidemiol 1994;8:173-187.
  18. Ekwo EE, Grusslink CA, Moawad A. Previous pregnancy outcomes and subsequent risk of premature rupture of amniotic sac membranes. Brit J Obstet Gynecol 1993;100(6):536-541.
  19. Algert C, Roberts C, Adelson P, Frammer M. Low birth weight in New South Wales, 1987: a Population-Based Study. Aust New Zealand J Obstet Gynaecol 1993;33:243-248.
  20. Lumley J. The epidemiology of preterm birth. Bailliere’s Clin Obstet Gynecology. 1993;7(3):477-498.X
  21. Chumnijarakij T, Nuchprayon T, Chitinand S., Onthuam N, et al. Maternal risk factors for low birth weight in Thailand. J Med Assoc Thai 1991;75(8):445-452.
  22. Mandelson MT, Maden CP, Daling JR. Low Birth Weight in  Relation Multiple Induced Abortions. Am J Public Health 1992;82; 391-394. ncbi.nlm.nih.gov/pmc/articles/PMC1694356/pdf/amjph00540-0065. pdf.
  23. Gong JH. Preterm delivery and its risk factors. Zhounghua Fu Chan Ke Za Chi Jan. 1992;27(1):22- 24.
  24. Michielutte R, Ernest JM, Moore ML, Meis PJ, Sharp PC, Wells HB, Buescher PA. A Comparison of Risk Assessment Models for Term and Preterm Low Birthweight. Preventive Medicine 1992;21:98-109.X
  25. Zhang J, Savitz DA. Preterm Birth Subtypes among Blacks and Whites. Epidemiology 1992;3:428-433. X^^
  26. Pickering RM, Deeks JJ. Risks of Delivery during 20th to the 36th Week of Gestation. Intl. J Epidemiology 1991;20:456-466.
  27. McGregor JA, French J, Richter R. Antenatal microbiologic and maternal risk factors associated with prematurity. Amer J Obstet Gynecol 1990;163:1465-1473.
  28. Harger JH, Hsing AW, Tuomala RE, Gibbs RS, et al. Risk factors for preterm premature rupture of fetal membranes: A multicenter case-control study. Am J Obstet Gynec 1990;163:130-137.
  29. Li YJ, Zhou YS. study of factors associated with preterm delivery. Zhongjua Liu Xing Bing Xue Za Chi. Aug 1990;11(4):229-234.
  30. Vasso L-K, Chryssa T-B, Golding J. Previous obstetric history and subsequent preterm delivery in Greece. European J Obstetrics & Gynecology Reproductive Biology 1990;37: 99-109.^^

1980-1989

  1. Mueller-Heubach E, Guzick DS. Evaluation of risk scoring in a preterm birth prevention study of indigent patients. Am J Obstetrics & Gyn 1989;160:829-837.^^
  2. Zwahr, C, Neubert D, Triebel U,Voight M, Kruppel KH. Correlation between some environmental, anamnestic and social markers of pregnant patients and the delivery of autotrophic premature and hypotrophic newborn infants. Zentralbl Gynakol 1988;110:479-487.
  3. Seidman DS, Ever-Hadani P, Slater PE, Harlap S, et al. Child-bearing after induced abortion: reassessment of risk. J Epidemiology Community Health 1988;42:294-298. jech.bmj.com/content/42/3/294.full.pdf.
  4. Main DM, Richardson D, Gabbe SG, Strong S, Weller SC, Prospective Evaluation of a Risk Scoring System for Predicting Preterm Delivery in Black Inner City Women. Obstetrics & Gynecology 1987;69:61-66.
  5. Krasomski G, Gladysiak A, Krajerski J. Fate of subsequent pregnancies after induced abortion in primiparae. Wiad Lek 1 December 1987;40(23):1593-1595.
  6. Lieberman E, Ryan KJ, Monson RR, Schoenbaum SC. Risk Factors Accounting For Racial Differences in the rate of premature birth. NEJM 1987;317:743-748.
  7. Ross MG, Hobel CJ, Bragenier JR, Bear MB, Bemis RL. A simplified risk-scoring system for prematurity. Amer J Perinatology 1986;3:339-344.
  8. Peterlin A Ardolsek L. The effect of induced abortion in adolescence on the manifestations of spontaneous abortion, premature abortion, and birth weight. Jugosl Ginekol Perinatol May-Aug 1986;26(3- 4):49-52.
  9. Shiono PH, Lebanoff MA. Ethnic Differences and Very Preterm Delivery. Am J Public Health 1986;76:1317-1321. ncbi.nlm.nih.gov/pmc/articles/PMC1646746/pdf/amjph00274-0055.pdf.^^
  10. Lumley J. Very low birth-weight (less than 1500g) and previous induced abortion: Victoria 1982- 1983. Aust NZ J Obstet Gynecol 1986;26:268-272.^^
  11. Pickering RM, Forbes J. Risk of preterm delivery and small-for-gestational age infants following abortion: a population study. British J Obstetrics and Gynecology 1985;92:1106-1112.
  12. Schuler D, Klinger A. Causes of low birth weight in Hungary. Acta Paediatrica Hungarica 1984;24:173-185.
  13. Zwahr C, Voigt M. The effect of various parameters on the incidence of premature births. Zentralbl Gynakol 1983;105:1307-1312.
  14. Puyenbroek J, Stolte L. The relationship between spontaneous and induced abortions and the occurrence of second-trimester abortion in subsequent pregnancies. Eur J Obstet Gynecol Reprod Biol 1983;14:299-309. ^^
  15. Pompe-Tansek NM, Andolsek L, Tekovcic B. Jugosl Ginekol Opstet Sept.-Dec. 1982;22(5-6):118- 120.
  16. Schoenbaum LS, Monson RR. No association between coffee consumption and adverse outcomes of pregnancy. N Engl J Med 1982;306:141-145.
  17. Madore C, Hawes WE, Many F, Hexter AC. A study on the effects of induced abortion on subsequent pregnancy outcome. Amer J Obstet Gynecol 1981;139(5):516-521
  18. Lampe LG, Ratar I, Bernard PP, et al. Effects of smoking and of induced abortion on pregnancy outcome. IPPF Med Bull 1981;15:3.
  19. Berkowitz GS. An Epidemiologic Study of Preterm Delivery. American J Epidemiology 1981;113:81- 92.^^
  20. Lerner RC, Varma AO. Prospective study of the outcome of pregnancy subsequent to previous induced abortion. Final report, Contract no. (N01-HD-62803). New York: Downstate Medical Center, SUNY, January 1981.
  21. Slater PE, Davies AM, Harlap S. The Effect of Abortion Method on the Outcome of Subsequent Pregnancy. J Reprod Med 1981;28:123-128.
  22. Obel EB. Long-Term sequelae following legally induced abortion. Danish Medical Bulletin 1980;27(2):61-74.^^
  23. Legrillo V. Quickenton P, Therriault GD, et al. Effect of induced abortion on subsequent reproductive function. Final report to NICHD. Albany, NY: New York State Health Department, 1980.
  24. Levin A, Schoenbaum S, Monson R, Stubblefield P, Ryan K. Association of Abortion With Subsequent Pregnancy Loss. JAMA 1980;243(24):2495-2499^^
  25. Kreibich H, Ludwig A. Early and late complications of abortion in juvenile primigravidae (including recommended measures). Z Arztl Fortbild (Jena) 1980;74(7):311-316.
  26. Zwahr C, Voigt M, Kunz L, et al. Relationships between interruption abortion, and premature birth and low birth weight. Zentrabl Gynaekol 1980;102:738-747.

1970-1979

  1. von Lembrych, S.: Schwangerschafts – Geburts – und Wochenbett verlauf nach kunsticher Unterbrechung der ersten Graviditat. Zentrabl Genaecol 1972; 94:164.
  2. Zwahr C, Coigt M, Kunz L, Thielemann F, Lubinski H. Multidimensional investigations to elucidate relationships between case histories of interrupted pregnancies and premature deliveries and low birth weight. Zentrabl Gynekol 1979;101(23):1502-1509
  3. Renkieleska M. Obstetrical complications in induced abortions. Ginekol Pol 1978;49:389-393.
  4. Macku F, Rokytova V, Titmann O. Artificial Interruption of Pregnancy in Primigravidae as a risk factor in future pregnancies. Cesk Gynekol 1978;43(5):340-343.
  5. Kreiblich H, Ehring E. Zentralfl Gynokol Effect of abortion on subsequent fertility with special reference to the abortion process. Zentralbl Gynakol 1978;100(19):1254–1260.
  6. Knarre P. Influence of abortions and interruptions of pregnancy in subsequent deliveries. II Cause of labor. Zentrabl Gynekol 1976;98(10):591-594.
  7. Link M, Wichmann A. Pregnancy in adolescents. Zentrabl Gynekol 1976;98(11):682-689.
  8. Mikolas M. The effect of the legalization of abortion on public health and some of its social concomitants in Hungary. Demografia 1973;16:70-113.
  9. Pohanka O, Balogh B, Rutkovszky M. The impact of abortion on the birth weight of newborns. Orb Hetil 1975;116:1983-1989.
  10. Fredrick J. Antenatal identification of women at high risk of spontaneous preterm birth. BJOG 1976;83:351-354.
  11. Chabada J, Pontuch A, Sutta I, Pohlova G. Interruptions of gravidity as a cause of premature labour Cesk Gynekol 1974;49(5):329-330.
  12. Kaminski M, Goujard J, Rumeau-Roquette. Prediction of low birthweight and prematurity by a multiple regression analysis with maternal characterisitics known since the beginning of the pregnancy. Intl J Epidem 1973;2:195-204.
  13. Dziewulska W. Abortion in the past versus the fate of the subsequent pregnancy. State of the newborn. Ginekol Pol 1973;44:1143-1148.
  14. Czeizel A, Bognar Z, Tusnady G, et al. Changes in mean birth weight and proportion of low-weight births in Hungary. Br J Prev Soc Med 1970;24:146-153.
  15. Hungarian Central Statistical Office. The effect of the number of abortions on premature births and perinatal mortality in Hungary. Budapest: 1972.
  16. World Health Organization. Special Programme of Research, Development and Research Training in Human Reproduction: Seventh Annual Report, Geneva, Nov. 1978.
  17. Lean TH, Hogue CJR, Wood J. Low birth weight after induced abortion in Singapore, Presented at the 105th Annual Meeting of the Americal Public Health Association, Washington DC, Oct. 31, 1977.
  18. Koller O, Eikhom SN. Late Sequelae of Induced Abortion in Primigravidae. Acta Obstet Gynecol Scand 1977;56:311-317.
  19. Harlap S, Davies AM. Late sequelae of induced abortion: Complications and Outcome of Pregnancy and Labor. Amer J Epidemiology 1975;102:219-224.
  20. Roht LH, Aoyama H, Leinen GE, et al. The association of multiple induced abortions with subsequent prematurity and spontaneous abortion. Acta Obstet Gynaecol Jpn 1976;23: 140-145.
  21. Ratter G et al. Effect of Abortion on Maturity of Subsequent Pregnancy. Med J Australia June 1979: 479-480.
  22. World Health Organization Task Force on the Sequelae of Abortion. Gestation, birthweight and spontaneous abortion. Lancet 1979;1:142-145.
  23. Obel E, et al. Pregnancy Complications Following Legally Induced Abortion With Special Reference to Abortion Technique. Acta Obstet Gynecol Scand 1979;58:147-152.^^
  24. Grindel B, Lubinski H, Voigt M. Induced abortion in primigravidae and subsequent pregnancy, with particular attention of underweight. Zentralbl Gynaekol 1979;101:1009-1114.
  25. Bognar Z, Czeizel A. Mortality and Morbidity Associated with Legal Abortions in Hungary, 1960- 1973. AJPH 1976;66:568-575.^^
  26. Papaevangelou G, Vrettos AS, Papadatos D, Alexiou C. The Effect of Spontaneous and Induced Abortion on Prematurity and Birthweight. The J Obstetrics and Gynaecology of the British Commonwealth. May 1973;80:418-422.^^
  27. Richardson JA, Dixon G. Effect of legal termination on subsequent pregnancy. British Med J 1976;1:1303-1304.
  28. Van Der Slikke JW, Treffers PE. Influence of induced abortion on gestational duration in subsequent pregnancies. BMJ 1978;1: 270-272.
  29. Pantelakis SN, Papadimitriou GC, Doxiadis SA.Influence of induced and spontaneous abortions on the outcome of subsequent pregnancies. Amer J Obstet Gynecol. 1973;116: 799-805.
  30. Dolezal A, Andrasova V, Tittlbachova S, et al. Interruption of pregnancy and their relation to premature labous and hyptrophic foetuses. Cesk Gynekol 1970:36:331.
  31. Drac P, Nekvasilova Z. Premature termination of pregnancy after previous interruption of pregnancy. Cesk Gynekol 1970;35: 332-333.

1960-1969

  1. Arvay A, Gorgey M, Kapu L. La relation entre les avortements (interruptions de la grossesse) et les accouchements prematures. Rev Fr Gynecol Obstet 1967;62:81-86
  2. Furusawa Y, Koya Y. The Influence of artificial abortion on delivery. In: Koya Y, ed. Harmful effects of induced abortion. Tokyo: Family Planning Federation of Japan,1966:74-83.
  3. Miltenyi K. On the effects of induced abortion. Demografia 1964;7:73-87.
  4. Barsy G, Sarkany J. Impact of induced abortion on the birth rate and infant mortality. Demografia 1963;6:427-467.

Appendix B: Studies on Surgical Abortion and Very Preterm Birth

XDenotes studies that included miscarriages and stillbirths as well as surgical abortions but did not report separate PTB/LBW risks
^^ Denotes studies that found dose/response (the more abortions, the higher the risk)

  1. Watson LF, Rayner J-A, King J, Jolley D, Forster D, Lumley J. Modelling prior reproductive history to improve predication of risk for very preterm birth. Paediatric Perinatal Epidemiology 2010;24:402-415.^^
  2. Watson LF, Rayner J-A, King J, Jolley D, Forster D, Lumley J. Modelling sequence of prior pregnancies on subsequent risk of very preterm birth. Paediatric and Perinatal Epidemiology 2010;24:416-423. ^^
  3. Reime B, Schuecking BA, Wenzlaff P. Reproductive Outcomes in Adolescents Who Had a Previous Birth or an Induced Abortion Compared to Adolescents’ First Pregnancies. BMC Pregnancy and Childbirth 2008;8:4
  4. Voigt M, Olbertz D, Fusch C, Krafczyk D. Briese V, Schneider KT. The influence of previous pregnancy terminations, miscarriages, and still-birth on the incidence of babies with low birth weight and pre- mature births as well as somatic classification of newborns. Z Geburtshilfe Neonatol 2008;212:5-12.^^
  5. Smith GCS, Shah I, White IR, Pell JP, Crossley JA, Dobbie R. Maternal and biochemical predictors of spontaneous preterm birth among nulliparous women: a systematic analysis in relation to degree of prematurity. International J Epidemiology 2006;35(5): 1169-1177.
  6. Stang P, Hammond AO, Bauman P. Induced Abortion Increases the Risk of Very Preterm Delivery; Results from a Large Perinatal Database. Fertility Sterility. Sept 2005;S159.
  7. Moreau C, Kaminski M, Ancel PY, Bouyer J, et al. Previous induced abortions and the risk of very preterm delivery: results of the EPIPAGE study. British J Obstetrics Gynaecology 2005;112(4): 430-437. ^^
  8. Ancel PY, Lelong N, Papiernik E, Saurel-Cubizolles MJ, Kaminski M. History of induced abortion as a risk factor for preterm birth in European countries: results of EUROPOP survey. Human Reprod 2004;19(3):734-740.
  9. Ancel PY, Saurel-Cubizolles M-J, Renzo GCD, Papiernik E, Breart G. Very and moderate preterm births: are the risk factors different? British J Obstetrics Gynaecology 1999;106:1162-1170.^^
  10. Zhou W, Sorenson HT, Olsen J. Induced Abortion and Subsequent Pregnancy Duration. Obstetrics & Gynecology 1999;94:948-953.^^
  11. Martius JA, Steck T, Oehler MK, Wulf K-H. Risk factors associated with preterm (<37+0 weeks) and early preterm (<32+0 weeks): univariate and multi-variate analysis of 106 345 singleton births from 1994 statewide perinatal survey of Bavaria. European J Obstetrics Gynecology Reproductive Biology 1998;80:183-189.^^
  12. Lumley J. The association between prior spontaneous abortion, prior induced abortion and preterm birth in first singleton births. Prenat Neonat Med 1998;3:21-24.^^
  13. Lumley J. The epidemiology of preterm birth. Bailliere’s Clin Obstet Gynecology 1993;7(3):477- 498. ^^
  14. Algert C, Roberts C, Adelson P, Frammer M. Low birth weight in New South Wales, 1987: a Population-Based Study. Aust New Zealand J Obstet Gynaecol 1993;33:243-248.^^
  15. Zhang J, Savitz DA. Preterm Birth Subtypes among Blacks and Whites. Epidemiology 1992;3:428- 433. X^^
  16. Mueller-Heubach E, Guzick DS. Evaluation of risk scoring in a preterm birth prevention study of indigent patients. Amer J Obstetrics & Gynecol 1989;160:829-837.^^
  17. Lumley J. Very low birth-weight (less than 1500g) and previous induced abortion: Victoria 1982- 1983. Aust NZ J Obstet Gynecol 1986;26:268-272.^^
  18. Schuler D, Klinger A. Causes of low birth weight in Hungary. Acta Paediatrica Hungarica 1984;24:173-185.
  19. Levin A, Schoenbaum S, Monson R, Stubblefield P, Ryan K. Association of Abortion With Subsequent Pregnancy Loss. JAMA 1980;243(24):2495-2499.^^
  20. Van Der Slikke JW, Treffers PE. Influence of induced abortion on gestational duration in subsequent pregnancies. BMJ 1978; 1:270-272.
  21. Watson LF, Rayner J-A, Forster D. Identifying risk factors for very preterm birth: A reference for clinicians. Midwifery 2012.
  22. Bhattacharya S, Lowit A, Bhattacharya S, Raja EA, Lee AM, Mahmood T, Templeton A. Reproductive outcomes following induced abortion; a national register-based cohort study in Scotland. BMJ OPEN 2012;2:e000911. bmjopen.bmj.com/content/2/4/e000911.full.pdf.
  23. Klemetti R, Gissler M, Niinimaki M, Hemminki E. Birth outcomes after induced abortion: a nationwide register-based study of first births in Finland. Human Reproduction 2012 August 29 ncbi.nlm.nih.gov/pubmed/22933527.
  24. Ghislain Hardy, Alice Benjamin, Haim A. Abenhaim. Effects of Induced Abortions on Early Preterm Births and Adverse Perinatal Outcomes. Journal of Obstetrics and Gynaecology Canada 2013;35(2):138- 143. jogc.com/abstracts/201302_Obstetrics_5.pdf
  25. Raisanen S, Gissler M, Saari J, Kramer M, Heinon S. Contribution of Risk Factors to Extremely, Very and Moderately Preterm Term Births- Register-Based Analysis of 1,390,742 Singleton Births. PLOS ONE April 2013;8(4):1-7. plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0060660
  26. Scholten BL, Page-Christiaens CML, Franx A, Hukkelhoven CWPM, Koster MPH. The influence of pregnancy termination on the outcome of subsequent pregnancies: a retrospective cohort study. BMJ OPEN 2013;3;e002803 doi:10.1136/bmjopen-2013-002803. bmjopen.bmj.com/content/3/5/e002803. full.pdf.html
  27. Usynina AA, Postoes VA, Grjiborski AM, Krettek A, Nieboer E, et al. Maternal Risk Factors for Preterm Birth in Murmansk County, Russia: A Register-Based Study. Paediatric Perinatal Epidemiology May 2016 [Population: Russian; Extremely-Preterm- Birth/IA O.R. 1.96 (1.32-2.91).
  28. Situ KC, Gissler M, Virtanen SM, Klemetti R. Risk of Adverse Perinatal Outcomes after Repeat Terminations of Pregnancy by their Methods: a Nationwide Register-based cohort study. Paediatric Perinatal Epidemiology August 2017. onlinelibrary.wiley.com/doi/10.1111/ppe.12389/full.
  29. Innes KE, Byers TE. First pregnancy characteristics and subsequent breast cancer risk among young women. International Journal of Cancer 2004;112(2):306-311. onlinelibrary.wiley.com/doi/10.1002/ ijc.20402/full.
  30. Melbye M, Wohlfahrt J, Andersen A-MN, Andersen PK. Preterm delivery and risk of breast cancer. British Journal of Cancer 1999;80(3/4):609-613. ncbi.nlm.nih.gov/pmc/articles/PMC2362328/ pdf/80-6690399a.pdf.
  31. Swingle HM, Colaizy TT, Zimmerman MB, et al Abortion and the risk of subsequent preterm birth: a systematic review and meta-analysis. J Reproductive Med 2009;54:95-108. johnrodgerssmith.com/Med- icalObservations/Swingle/JRM%20Swingle%20paper%202009.pdf.
  32. Lemmers M, Vershoor MA, Hooker AB, Opmeer BC, Limpens J, Huirne JA, Ankum WM, Mol BW. Does dilation and curettage (D & C) increase the risk of preterm birth in subsequent pregnancies? A systematic review and meta-analysis.

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American Association of Pro-Life Obstetricians & Gynecologists

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