Our objective was to study the effects of maternal body mass index (BMI) on the mode of delivery for primigravid and multigravid women. A retrospective cohort study was conducted at the University College Hospital Galway, Ireland, of 5162 women delivered from 2001 to 2003. BMI at the first antenatal appointment was calculated. Comparisons were made between each of the five BMI categories separately for primigravid and multigravid women in relation to gestation at delivery, age, mode of delivery, and birthweight. There were 5162 deliveries during the time period of the study; 2006 were primigravid and 3156 were multigravid women. Overall, 2.6% of women were underweight, 49.2% were normal weight, 22.8% were overweight, 19.8% were obese, and 5.6% were morbidly obese. In comparison with women of normal weight, for overweight and obese women, there was a progressive reduction in vaginal delivery rate with increasing BMI. For morbidly obese primigravida, this reduction was by 33.5% (from 83.1% to 55.3%, chi(2) = 39.84, p < 0.001), and for multigravida was by 23.6% (from 86% to 65.7%, chi(2) =53.05, p < 0.001). Obesity conferred a two- to threefold increased risk of delivery by emergency caesarean section for both primigravid (obese, relative risk [RR] 2.16, 95% confidence interval [CI] 1.72 to 2.73; morbidly obese, RR 2.30, CI 1.61 to 3.37) and multigravid women (obese, RR 1.97, CI 1.45 to 2.67; morbidly obese, RR 2.44, CI 1.61 to 3.69). We concluded that increasing maternal BMI exerts a progressive adverse effect on vaginal delivery rates for both primigravid and multigravid women. Obese primigravida should be counseled antenatally about the 30% risk of emergency caesarean section.