Background. To investigate the association between maternal body mass index (BMI) at delivery and the risk of caesarean section due to dystocia during the first stage of labour in low risk pregnancies. Methods. Historical cohort study that included 6,949 low risk women who delivered at the Medical University of South Carolina from 1994 to 2004, presenting a singleton birth at term, and a vaginal delivery attempt by spontaneous labour. Women presenting large for gestational age newborns were excluded. Adjusted odds ratios (ORs) for caesarean section due to dystocia and for caesarean section due to other reasons were estimated using a multinomial regression logistic model and compared using the Wald's test. Results. Women with a BMI > = 30 kg/m2, of maternal age > = 30 years and nulliparas had an increased rate of caesarean section delivery either due to dystocia or for other reasons. Newborn weight > = 3,500 g, races other than Caucasian, age between 25 and 29 years, BMI between 25 and 29.9 kg/m2, and fetal membranes rupture more than 24 h before the onset of the labour were associated with an increased rate of caesarean section due to dystocia only. On the contrary, newborn weight between 2,500 and 2,999 g was associated with a significant decrease in the rate of caesarean section due to dystocia. Newborn weight <3,000 g was associated with a risk for caesarean section due to other reasons. The population risk for caesarean section due to dystocia, attributable to BMI > = 35 kg/m2 in low risk pregnancies, was 13.3%. Conclusions. An increased maternal BMI was associated with the risk of caesarean section due to dystocia. This information should be made available to women who are overweight or obese at antenatal booking or at the first trimester visit.