After Abortion, Danish Women's Odds of Preterm Delivery Are Doubled

Volume 32, No. 4, July/August 2000

This article is also available in PDF format [PDF]

DIGEST

After Abortion, Danish Women's Odds of Preterm Delivery Are Doubled

Danish women whose first pregnancy ended in abortion are about twice as likely as those who did not terminate their first pregnancy to subsequently deliver an infant at less than 37 weeks' gestation, according to results of a population-based cohort study; they have a somewhat elevated risk of having a subsequent delivery at 42 or more weeks of gestation.¹Analyses of the same cohort also suggest that women who have undergone abortion have twice the risk of other women of later bearing a low-birth-weight infant.²

The investigators used three national registries to identify Danish women who had a first pregnancy in 1980-1982, determine the outcomes of these pregnancies and gather information on the women's additional pregnancies until 1994. Their analyses compare two groups of women who had at least two pregnancies during the study period: 15,727 who terminated their first pregnancy and 46,026 who had other outcomes.

Most of the women in the abortion group (83%) had only one abortion before having a live birth; 15% had two, and the rest had three or more. At the time of their last abortion, 43% of women in this group were younger than 20, and 52% were aged 20-29. All abortions were performed during the first trimester; 92% were vacuum aspiration procedures, and 7% were done by dilation and evacuation.

For their study of the effects of abortion on subsequent pregnancy duration, the researchers used logistic regression techniques to examine the likelihood of preterm delivery (before 37 weeks) and of postterm delivery (42 weeks or later), controlling for potentially confounding factors. They also analyzed the data according to the woman's number of pregnancies, up to and including one that ended in a live singleton birth, and the number of months between that pregnancy and the previous one.

When age, residence, interpregnancy interval and number of previous miscarriages were taken into account, women with one previous abortion were 1.9 times as likely as women in the comparison group, those with two previous abortions were 2.7 times as likely and those with three or more previous abortions were 2.0-2.2 times as likely to have a preterm birth. In general, the risk varied slightly according to the method of abortion used, but it was sharply higher (odds ratio, 12.6) among women who had had two abortions by dilation and evacuation. Increases in the risk of preterm birth were significant mainly among women whose interpregnancy interval was 12 months or more; the pattern of risk among this subgroup was similar to the overall pattern. Previous abortion also was associated with a doubling of the odds of very preterm delivery (before 34 weeks' gestation).

Women who had had an abortion were at an increased risk of delivering postterm; the odds ratios increased from 1.3 among those who had had one abortion to 1.6 for those who had had three or more. The likelihood of postterm delivery was significantly elevated regardless of the interval between pregnancies. According to the investigators, these results were "unexpected" and might reflect a "chance association."

In the study of the influence of abortion on the risk of having an infant with a low birth weight (less than 2,500 g), the researchers conducted multivariate analyses including the newborn's gender, as well as mother's age, residence and interpregnancy interval, among the controls. They found that women who had had one or more abortions were 1.9 times as likely as those who had had none to deliver a low-birth-weight baby; the increased risk was restricted to those who had obtained vacuum aspiration procedures.

For women who had had one abortion, the odds of having a low-birth-weight baby were elevated at interpregnancy intervals of seven months or more, while for those who had had more than one abortion, the risk increased significantly only if the interval was longer than 12 months. Additional analyses showed that whether the abortion took place within the first eight weeks of gestation or within weeks 9-12, the risk of low birth weight increased if the interpregnancy interval was seven months or longer.

The investigators note that earlier research on the effects of abortion on subsequent pregnancy outcomes has yielded conflicting results and that the mechanisms by which abortion may influence the duration of later pregnancies or infants' birth weight are unclear. Furthermore, they acknowledge that they were unable to control for a number of factors that may contribute to the outcomes they studied, including women's smoking habits, birth weight, race, socioeconomic status and pregnancy-related medical conditions. However, they point out that their study has advantages over previous work because of its size, because it was population-based and because of the completeness of abortion reporting in Denmark.--D. Hollander

References
1. Zhou W, Sorensen HT and Olsen J, Induced abortion and subsequent pregnancy duration, Obstetrics & Gynecology, 1999, 94(6):948-953.

2. Zhou W, Sorensen HT and Olsen J, Induced abortion and low birth weight in the following pregnancy, International Journal of Epidemiology,2000, 29(1):100-106.

Guidelines for Authors

Statement of Accuracy/User Agreement