Prepregnancy body mass index and the length of gestation at term

Objective: The purpose of this study was to examine the relationship between prepregnancy body mass index (BMI) and length of gestation at term. Study Design: This was a retrospective study of 9336 births at the University of California, San Francisco, at ≥37 weeks' gestation. We performed univariate and multivariable analyses of the associations between prepregnancy BMI and length of gestation (≥40, ≥41, and ≥42 weeks' gestation). Results: Overweight women were more likely to deliver at ≥40, ≥41, and ≥42 weeks' gestation than were women who were underweight or normal weight. In multivariable analyses, higher prepregnancy BMI was associated with higher risk of progressing past 40 weeks. Obese women had 69% higher adjusted odds of reaching 42 weeks' gestation, compared with women of normal prepregnancy BMI (adjusted odds ratio, 1.69; 95% confidence interval, 1.23-2.31). Conclusion: Higher BMI is associated with prolonged gestation at term. Achieving optimal BMI before conception may reduce the risk of postterm pregnancy and its associated complications.