Women with two closely spaced pregnancies risk early delivery, especially if the first ended in a term birth

Danish women who became pregnant within eight months after giving birth had an elevated risk of delivering preterm, and the relative risk was higher if the women had delivered the previous baby at term than if they had given birth early. However, short interpregnancy intervals were not associated with women's odds of having a low-birth-weight baby.1

Using data from Denmark's national birth registry, analysts identified 10,187 women who had had two consecutive live, singleton births between 1980 and 1992. They conducted logistic regression analyses to examine the effect of short interpregnancy intervals on the risk of preterm delivery (i.e., before 37 weeks' gestation) and low birth weight (less than 2,500 g), controlling for a variety of potentially confounding factors.

The sample was composed predominantly of women older than 25. About two in five women were categorized as having an intermediate social status (on the basis of the job they or their husband held), and the rest were evenly divided between low and high social status. Women whose interpregnancy interval was eight months or less were younger, more likely to have had previous births and of lower social status than those with a longer interval between pregnancies.

In initial analyses, those with interpregnancy intervals of eight months or less had significantly elevated odds of poor outcomes. Compared with women whose pregnancies were 2-3 years apart, those who became pregnant again four months or less after giving birth had 3-4 times the risk of having a preterm birth or low-birth-weight baby (odds ratios, 3.0 and 3.7, respectively). Those who became pregnant within the next four-month interval (i.e., 4-8 months after giving birth) had somewhat lower but still significantly elevated risks of these outcomes (1.8 and 2.3, respectively).

When the analyses took into account the gestational age of the second infant studied and the mother's age, parity and social status, the association between short interpregnancy intervals and low birth weight was no longer statistically significant, but an elevated risk of preterm birth remained. Women who became pregnant in four months or less and those who conceived 4-8 months after giving birth were, respectively, 3.6 and 2.3 times as likely to deliver preterm as were those whose interpregnancy interval was 2-3 years.

The odds of a preterm birth were similar if women had previously delivered at 39-40 weeks' gestation, but they were lower and no longer statistically significant among those who had delivered at shorter gestations. Additionally, analyses including only women whose first infant was born at term, was not low-birthweight or small for gestational age, and survived for at least one year confirmed that a short interval between pregnancies raised the odds of preterm birth but was not related to the risk of low birth weight. The analysts note that a mother who becomes pregnant shortly after having a term birth may not have sufficient time to replenish lost nutrients.

The researchers caution that the National Board of Health made only 5% of the population's data available for study, and confidence intervals for the results were wide. Also problematic was the possible misclassification of gestational age. Nevertheless, they contend, short interpregnancy intervals are associated with preterm birth. The potential risk, they conclude, should be considered when women plan pregnancies, "because prematurity is one of the most important causes of infant death and impaired child development in the industrialized countries."-E. Bresler

Reference

1. Basso O et al., Low birth weight and preterm birth after short interpregnancy intervals, American Journal of Obstetrics and Gynecology, 1998,178(2):259-263.

Copyright The Alan Guttmacher Institute Sep/Oct 1998
Provided by ProQuest Information and Learning Company. All rights Reserved