Body mass index (BMI) is widely used to categorize the degree of obesity and to guide recommendations for weight gain during pregnancy, to examine the relationship between the maternal body mass index of nulliparous women and the route of delivery in a sample of Iranian Kurdish women, a cross sectional descriptive study was conducted, at Beasat Hospital in Sanandaj, the capital of Kurdistan province, west of Iran. The study sample consisted of 980 nulliparous women with spontaneous labor. Results showed that there is a significant association between cesarean section and higher maternal BMI. The cesarean section rate rose from 30% in women with normal BMI to 56% in the women with BMI≥ 35. Overweight women should be given information about risk of cesarean section before conception and be encouraged to reduce their weight.