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 > or = 37 weeks' gestation. We performed univariate and multivariable analyses of the associations between prepregnancy BMI and length of gestation (> or = 40, > or = 41, and > or = 42 weeks' gestation).

RESULTS: Overweight women were more likely to deliver at > or = 40, > or = 41, and > or = 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.