OBJECTIVE:To assess the association between pre-gestational obesity and weight gain with cesarean delivery and labor complications. METHODS:A total of 4,486 women 20-28 weeks pregnant attending general prenatal care clinics of the national health system in Brazil from 1991 to 1995 were enrolled and followed up through birth. Body mass index categories based on prepregnancy weight and total weight gain were calculated. Associations between body mass index categories and labor complications were adjusted through logistic regression analysis. RESULTS:Obesity was present in 308 (6.9%) patients. Cesarean delivery was performed in 164 (53.2%) obese, 407 (43.1%) pre-obese, 1,045 (35.1%) normal weight and 64 (24.5%) underweight women. The relative risk for cesarean delivery in obese women was 1.8 (95% CI: 1.5-2.0) compared to normal weight women. Greater weight gain was particularly associated with cesarean among the obese (RR 4 th vs 2 nd weight gain quartile 2.2; 95% CI: 1.4-3.2). Increased weight at the beginning of pregnancy was associated with a significantly higher adjusted risk of meconium with vaginal delivery and perinatal death and infection in women submitted to cesarean section. Similarly, greater weight gain during pregnancy increased the risk for meconium and hemorrhage in women submitted to vaginal delivery and for prematurity with cesarean. CONCLUSIONS:Pre-gestational obesity and greater weight gain independently increase the risk of cesarean delivery, as well as of several adverse outcomes with vaginal delivery. These findings provide further evidence of the negative effects of prepregnancy obesity and greater gestational weight gain on pregnancy outcomes. INTRODUCTIONThe prevalence of overweight and obesity have substantially increased in the last decades, especially in women. Data from 1989 reported that 13.3% of Brazilian women were obese and a national survey conducted in 1996 showed that 10.2% of Brazilian women of reproductive age (20 to 49 years) were obese. Additionally the prevalence of overweight among Brazilian women in this age group was 36.8% in 1996. 13Medical complications such as diabetes and hypertension are significantly increased during pregnancy in obese patients. Fetal distress, meconium staining, arrest disorders, hemorrhage, difficult peridural access and operatory time are also increased in obese women. Furthermore, evidence of increased risk for cesarean section, endometritis, prolonged hospital stay, thromboembolic phenomena, anemia, urinary tract infection, prematurity and fetal demise have been described. 23Cesarean sections (C-sections) have become more frequent since 1970-80 around the world. Between 1965 and 1986, the rate of cesarean delivery in the United States increased from 4.5 to 24.1%. 21 From 1984 to1993, cesarean delivery was the most frequent major surgical procedure in the United States. Rates then declined slightly to 20.7% in 1996 and, more recently increased again in 2002 reaching 26.1%, the highest rate ever recorded.12 In a city of southern Brazi...