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.