Contribution of excess weight gain during pregnancy and macrosomia to the cesarean delivery rate, 1990-2000.

OBJECTIVE After declining for many years, cesarean delivery rates recently increased. To explore whether this increase is associated with excess weight gain during pregnancy, resulting in macrosomic infants who require cesarean delivery, we examined trends in excess weight gain, macrosomia, and cesarean delivery. METHODS Analysis of 1990-2000 US Natality Files of birth certificate data were restricted to first birth, singleton infants of 37 to 42 weeks' gestation to avoid confounding by repeat cesarean delivery, complications of multigestational pregnancy, and preterm and postterm birth. Excess weight gain was defined according to current guidelines (41+ lb) and macrosomia as birth weight >4000 g. RESULTS From 1990-2000, excess weight gain rose steadily from 18.6% to 24.2%. There was a 19.3% decline in macrosomic infants among women who gained excess weight compared with an 11.9% decline among women who gained 15-40 lb, although the absolute risk remained substantially greater among women who gained excess weight (eg, 14.2% vs 7.2%, in 2000). From 1990-1997, cesarean delivery declined by 20.2% among women who gained excess weight compared with 15.7% among women who gained 15 to 40 lb. After 1997, cesarean delivery increased in all weight gain categories, and absolute risks in 2000 were 25.8% for women who gained excess weight compared with 21.6% for women who gained 15-40 lb. Overall, women who gained excess weight accounted for 24.1% of cesarean deliveries in 1990 and 28.1% in 2000. CONCLUSIONS Excess weight gain and macrosomia do not seem to be the primary factors that contribute to the recent increase in cesarean delivery because cesarean delivery rates have increased in all weight gain categories and macrosomia rates have decreased steadily from 1990-2000. Nonetheless, women who gain excess weight account for a growing proportion of cesarean deliveries because their relative numbers have grown.