Interpregnancy interval and risk of preterm labor.

In 1977-1980, over 12,000 pregnant women being followed at the Boston Hospital for Women were interviewed and had their medical records reviewed. The effect of interpregnancy interval on the risk of preterm labor was estimated in 4,467 of these women whose previous pregnancy had resulted in a full-term live birth. The rate of preterm birth after the spontaneous onset of labor in this cohort was 3.8 percent. While the possibility of an increased risk of preterm labor for interpregnancy intervals of 3 months or less cannot be definitely excluded (adjusted odds ratio = 2.0, 95 percent confidence interval 0.7-5.4), no relation was found between other interpregnancy intervals and the risk of preterm labor. Earlier work from this same cohort showed a strong negative association between interpregnancy interval and small-for-gestational-age birth. Combining this with the results from the present study reinforces the importance of differentiating low birth weight due to preterm birth from that due to intrauterine growth retardation.