Changes in prepregnancy body mass index between pregnancies and risk of preterm phenotypes.

We examined whether the risk of preterm birth and its subtypes (medically indicated and spontaneous preterm) are influenced by changes in prepregnancy body mass index (BMI) between the first and second pregnancies. A population-based, retrospective cohort analysis was performed using the Missouri (1978 to 2005) longitudinally linked cohort data sets. Women with their first two successive singleton live births ( N = 436,502) were analyzed. The risks for preterm birth and its subtypes were evaluated using multivariate logistic regression modeling. Compared with women who maintained normal interpregnancy BMI, women who lowered their weight from normal to underweight were more likely to experience preterm and spontaneous preterm birth (odds ratio [OR] 1.5, 95% confidence interval [CI] 1.4 to 1.6). Women with BMI changes from normal weight to obese (OR 1.4, 95% CI 1.2 to 1.6) and normal weight to overweight (OR 1.2, 95% CI 1.1 to 1.3) were at increased risk of medically indicated preterm birth. A decrease in prepregnancy BMI from normal to underweight is associated with increased risk of spontaneous preterm birth, and prepregnancy BMI increases from normal to overweight or to obese BMI are associated with increased risk of medically indicated preterm birth.