a The Danish Epidemiology Science Centre, Aarhus University, Vennelyst Boulevard 6, DK-8000 Aarhus C, Denmark.
b Shanghai Institute of Planned Parenthood Research, 2140 Xie Tu Road, Shanghai 200032, PR China.
c Department of Medicine V, Aarhus University Hospital, DK-8000 Aarhus C, Denmark.
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Abstract |
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Methods Cohort study using the Danish Medical Birth Registry (MBR), the Hospital Discharge Registry (HDR), and the Induced Abortion Registry (IAR). All women who had their first pregnancy during 19801982 were identified in the MBR, the HDR, and the IAR. We included all 15 727 women whose pregnancy was terminated by a first trimester induced abortion in the induced abortion cohort and 46 026 women whose pregnancy was not terminated by an induced abortion were selected for the control cohort. All subsequent pregnancies until 1994 were identified by register record linkage.
Results Low birthweight (<2500 g) in singleton term live births occurred more frequently in women with one, two, three or more previous induced abortions, compared with women without any previous induced abortion of similar gravidity, 2.2% versus 1.5%, 2.4% versus 1.7%, and 1.8% versus 1.6%, respectively. Adjusting for maternal age and residence at time of pregnancy, interpregnancy interval, gender of newborn, number of previous spontaneous abortions and number of previous low birthweight infants (control cohort only), the odds ratios (OR) of low birthweight in singleton term live births in women with one, two or more previous first trimester induced abortions were 1.9 (95% CI : 1.6, 2.3), and 1.9 (95% CI : 1.3, 2.7), respectively, compared with the control cohort of similar gravidity. High risks were mainly seen in women with an interpregnancy interval of more than 6 months.
Conclusions The findings suggest a positive association between one or more first trimester induced abortions and the risk of low birthweight in subsequent singleton term live births when the interpregnancy interval is longer than 6 months. This result was unexpected and confounding cannot be ruled out.
Keywords Induced abortion, low birthweight, interpregnancy interval, follow-up study, register linkage
Accepted 22 July 1999
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Introduction |
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Since the procedure is frequent we need to estimate its risks since even a small increase in long-term complications will have major public health importance. Risk estimates are also needed for decision-making when new induced abortion procedures are compared with standard methods.
In this study we will primarily focus upon low birthweight (LBW), i.e. <2500 g. It is an important measure since it is closely correlated with increased perinatal mortality11 and because the quality of birthweight data is high. Intrauterine fetal growth retardation is also associated with the long-term risk of coronary heart disease, stroke, diabetes and impaired cognitive function. Organ programming has been suggested as responsible for this increased susceptibility, but the mechanisms are unknown.1215 Fetal growth depends upon appropriate placental functioning,16 and it is possible that pregnancy termination interferes with placental development, perhaps related to trauma or infection.
There are many determinants of fetal growth,1730 and previous induced abortion is among these as concluded by some studies,25,31 but not all.26,3235 Some of the negative studies, however, did not have the power to detect a small or moderate increase in LBW risk. Whether induced abortion is a risk factor for LBW is still a controversial issue and most studies have been subject to recall and selection bias.
The aim of this study was to examine the association between induced abortion and the risk of a LBW infant in the subsequent pregnancy using a large study cohort with complete follow-up and with accurate data on induced abortion. We also wanted to consider the time lapsed from the induced abortion to the new conception (the interpregnancy interval). We expected the risk to be highest when the interpregnancy interval was short because traumas and infections will heal with time.
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Methods, Subjects and Data |
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We identified all who were registered as primigravidae (index pregnancy event) in the MBR and the HDR as well as in the IAR from 1980 to 1982. We identified all women with no prior registered pregnancies in the MBR, the HDR or the IAR up to 1980.
From this group we identified two cohorts: (a) The abortion cohort: All women whose pregnancies were terminated by a first trimester induced abortion (15 727 women). Induced abortions in the IAR and the HDR were identified as ICD-8 (640.00 642.99). We did not include abortions performed for medical reasons or abortions induced after the first trimester. (b) The control cohort: All primigravidae who had a spontaneous termination of their pregnancies; 41 076 women who experienced stillbirth or live birth and 4950 women who had spontaneous abortions. Spontaneous abortions in the HDR were identified as ICD-8 (643.00645.99).
All subsequent pregnancies were then identified by register record linkage in both cohorts up to 1994 by means of the MBR, the IAR, and the HDR. All live births and still births are registered in the MBR, all spontaneous abortions leading to hospitalization in the HDR (more than 80% of recognized abortions),39 and induced abortions in the IAR and the HDR.
Furthermore, we defined subcohorts in the abortion cohort of one, two, three or more induced abortions in a row before the woman had her first informative pregnancy (not terminated by induced abortion). In the control cohort, the informative pregnancy was all consecutive subsequent pregnancies which were not terminated by induced abortion. We focused only on the outcomes of the first informative pregnancy following the first trimester induced abortion. Only singletons were included in the analysis.
The LBW of term singleton live births (gestational age 3741 completed weeks) were mainly compared in the two cohorts considering gravidity. Analyses were stratified on interpregnancy intervals in order to study risk as a function of time lapsed since the induced abortion.
Maternal residence at the time of the last pregnancy was grouped into Copenhagen, cities with more than 100 000 inhabitants (Aarhus, Aalborg, Odense), and other regions.
We used logistic regression analyses (multiple regression analyses) in order to adjust for potential confounders such as, maternal age and residence at time of the informative pregnancy, number of previous LBW newborns, number of previous spontaneous abortions, and gender of newborn. Statistical analyses were performed using SPSS FOR WINDOWS (6.1.3 version).
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Results |
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Discussion |
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Our study is large, without non-responders, has complete follow-up and is carried out within a uniformly organized health care system and almost all induced abortions are reported to the IAR.40 The main shortcoming is the lack of data on confounders such as social factors and smoking, although we do not expect this type of confounding to be related to the choice of abortion method. Gestational age was recorded by the midwives and based upon ultrasound or the date of the last menstruation. In any case, estimations were made early postpartum and without access to data on the induced abortion.
Our findings of an increased risk is supported by other studies. Lekea-Karanika et al. found that mothers who had induced abortions had an increased relative risk (RR) of LBW in subsequent singleton births (RR = 1.8, 95% CI : 1.3, 2.3).25 Algert et al.26 found an unadjusted RR of small for gestational age in singleton births following induced abortion of 1.2 (95% CI : 1.1, 1.2), but after adjustment for 11 potential confounders the RR was even closer to one. A similar observation was made by Mandelson et al.32
After reviewing the literature, Hogue concluded that vacuum aspiration induced abortion does not protect against LBW in first-born offspring compared with women who had one previous delivery.33 Atrash et al. suggest that evacuation with dilatation is associated with an increased risk of LBW.41 In our study, both the vacuum aspiration method and the evacuation method were associated with an increased risk of LBW in singleton term live births. Although, the negative results after a short interpregnancy interval mitigate against causation, the results should be interpreted with caution.
Stratifying on interpregnancy interval may introduce confounding by fecundity and low fecundity has been associated with poor reproductive outcome. The negative findings after a short interval may be due to unequal fecundity between the compared groups, although it is difficult to predict the direction of the difference.
We know that gravity and parity are associated with birthweight26,2830,35 but it is unclear whether these associations are causal or whether they reflect differential pregnancy planning.42 Our cohorts start with primigravidae, and all women therefore have the same gravidity, but only those giving birth in the control cohort advance their parity. Comparing equal parity and gravidity thus requires use of different segments of the study base for comparison, and in most of our analyses we have used gravidity, by counting all abortions (induced and spontaneous) and births, as the point of reference.
Maternal age, previous spontaneous abortion, previous LBW, and gender of newborn are all known determinants of LBW,25,2830 which is also seen in our study. We had no data on smoking during pregnancy,17,20,28,29 mother's own birthweight,18,19 diet in pregnancy,18,30 maternal gestational weight gain,20,29,30 maternal working conditions during pregnancy,22 hypertensive disorders in pregnancy,23 or low socioeconomic status.30,43 It is likely that some of these risk factors will bias our result towards high OR.
Unfortunately the registries have no data on smoking habits and so standard adjustment for this confounder is therefore not an option. We can, however, estimate the confounding effect of smoking since we have data from a large cohort of Danish pregnant women from 1984 to 1987.44 In this study 43% of the pregnant women without a history of induced abortion smoked during pregnancy compared with 56% of the pregnant women with a history of induced abortion (data not shown). In this study, smoking was associated with an increased risk of LBW in singleton newborns of 36 weeks of gestation (OR = 2.8). If we assume a similar difference in smoking habits between our two cohorts and a similar association between smoking and LBW in singleton term live births, the OR for induced abortion and LBW should be reduced by a factor 1.13 due to confounding by smoking. We thus expect that the smoking adjusted OR for induced abortion is 1.77 instead of 2.00 found in the analyses without adjustment for smoking. We expect this adjustment to be quite accurate and find it unlikely that the association is entirely due to smoking.
We compare outcomes with similar gravidity but most of the women in the control cohort are of parity one and all exposed are of parity 0. We do this because gravidity is found to be equally associated with birthweight compared to parity.45 Furthermore, it is unclear if these associations are driven by selection rather than biological mechanisms.42
Our study did not show any strong association between induced abortion and LBW and it is possible that the association observed is due to confounding.
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Acknowledgments |
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References |
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