Gestational weight gain, macrosomia, and risk of cesarean birth in nondiabetic nulliparas.

OBJECTIVE: To examine how the association between excessive weight gain and cesarean birth is modified by infant birth weight in nondiabetic women.
METHODS: We designed a retrospective cohort study of singleton, term, nulliparous women with cephalic presentations delivering at a single university hospital. Subjects with diabetes were excluded. Bivariate and multivariate analyses were performed. Regression models controlled for maternal age, maternal prepregnancy body mass index, gestational age, ethnicity, smoking, birth weight, and date of delivery.
RESULTS: Women gaining above Institute of Medicine guidelines were more likely to have a cesarean birth, even if birth weight was less than 4,000 g. In the multivariate analysis, women with excessive weight gain had an odds ratio of 1.40 (95% confidence interval 1.22-1.59) for cesarean birth. When absolute weight gain (total pregnancy weight gain minus birth weight and placental weight) was used in the multivariate analysis, excessive weight gain was still an independent predictor of cesarean delivery. Although macrosomia was a stronger predictor of cesarean than weight gain alone, excessive weight gain was much more common than macrosomia in our cohort.
CONCLUSION: Excessive weight gain during pregnancy is an independent risk factor for cesarean birth, even when birth weight is not excessive. Other mechanisms besides macrosomia may be involved in the association between high weight gain and cesarean birth. We estimate that of the approximately 288,000 primary cesarean deliveries performed in nulliparas annually in the United States, 64,000 would be prevented if no women gained above Institute of Medicine recommendations. LEVEL OF EVIDENCE: II-2