|Objective To evaluate the risk of emergency cesarean section according to the prepregnancy body mass index (BMI) and gestational weight gain per the 2009 Institute of Medicine guidelines. Methods A retrospective analysis of data from 2,765 women with singleton full-term births (2009 to 2012) who attempted a vaginal delivery was conducted. Pregnancies with preeclampsia, chronic hypertension, diabetes, planned cesarean section, placenta previa, or cesarean section due to fetal anomalies or intrauterine growth restriction were excluded. Odds ratios (ORs) and confidence intervals (CIs) for emergency cesarean section were calculated after adjusting for prepregnancy BMI or gestational weight gain. Results Three-hundred and fifty nine (13.0%) women underwent emergency cesarean section. The adjusted OR for overweight, obese, and extremely obese women indicated a significantly increased risk of cesarean delivery. Gestational weight gain by Institute of Medicine guidelines was not associated with an increased risk of cesarean delivery. However, inadequate and excessive weight gain in obese women was highly associated with an increased risk of emergency cesarean section, compared to these in normal BMI (OR, 5.56; 95% CI, 1.36 to 22.72; OR, 3.63; 95% CI, 1.05 to 12.54; respectively), while there was no significant difference between normal BMI and obese women with adequate weight gain. Conclusion Obese women should be provided special advice before and during pregnancy for controlling weight and careful consideration should be needed at the time of vaginal delivery to avoid emergency cesarean section.