ObjectivesTo evaluate, in a French multicenter cohort, the risk of C-section based on a high pre-pregnancy body mass index (BMI). Secondary objectives were to assess the risk of elective C-section, severe post-partum hemorrhage (>1L), severe perineal tears (3rd and 4th degree) and neonatal complications according to pre-pregnancy BMI. Study designThis historical cohort study analyzed records from the French AUDIPOG perinatal database. Inclusion criteria were deliveries≥22 weeks (or with a birth weight≥500g). Women with BMI<18.5kg/m2 (n=31,766) were excluded. After these exclusions, the study sample included 314,851 women between 1999 and 2009. Patients were classified among four BMI subgroups (normal: 18.5–24.9kg/m2, overweight: 25–29.9kg/m2, class I and II obesity: 30–39.9kg/m2 and class III obesity:≥40kg/m2). BMI was calculated using pre-pregnancy self-reported weight. Results were expressed as crude and adjusted relative risks (aRR). ResultsA C-section occurred in 16.4%, 22.7%, 28.8% and 39.4% of normal BMI, overweight, obese and class III obese women, respectively (P<10−4). aRR of C-section increased with BMI: 1.26 [95%CI: 1.22–1.30] for BMI between 25–29.9kg/m2; 1.39 [95%CI: 1.34–1.45] for BMI between 30–39.9kg/m2 and 1.72 [95%CI: 1.57–1.90] for BMI≥40kg/m2; but not the elective C-section. Neonatal complications were more frequent with increasing maternal BMI (BMI 25–29.9: aRR=1.09 [95%CI: 1.06–1.12]; BMI 30–39.9: aRR=1.20 [95%CI: 1.16–1.25]; BMI≥40: aRR=1.33 [95%CI: 1.21–1.45]). ConclusionOur study confirmed that pre-pregnancy BMI is an important factor to consider because its elevation is associated with adverse obstetrical outcomes, especially cesarean delivery and neonatal complications.