Linear association between maternal pre-pregnancy body mass index and risk of caesarean section in term deliveries

Objective Maternal obesity is a well-known risk factor for caesarean delivery. The aim of this study is to determine whether all the spectrum of pre-pregnancy maternal corpulence (body mass index [BMI]) is associated with the risk of caesarean delivery. Design Observational study over 4.5 years (2001-05). Setting Groupe Hospitalier Sud-Reunion's maternity (island of La Reunion, French overseas department, Indian Ocean). Population All consecutive singleton live births having delivered at the maternity. Methods Data have been analysed according to different risk factors. Maternal corpulence has been defined as the maternal pre-pregnancy weight. BMIs have been studied by multiples of 5 kg/m(2) from 10-14.9 kg/m(2) to 40-44.9 kg/m(2). Main outcome measure Rate of caesarean section. Results There were 17 462 singleton live births during the period, of which 16 952 (97.1% of the total) pre-pregnancy BMIs have been determined. There is a linear association (chi(2) for linear trend, P < 0.001) between maternal corpulence and risk of caesarean deliveries, the leanest mothers having the best rate of vaginal delivery. This linear association exists in a model controlling for diagnosis of gestational diabetes, term deliveries (>= 37 weeks), very short maternal height (< 1.50 m), primiparity and maternal age >= 35 years (adjusted chi(2), P < 0.001). Conclusion There is a significant linear association between pre-pregnancy maternal corpulence and risk of caesarean deliveries in pregnancies at term. The authors discuss several interpretations including the adaptability of fetal birthweights to maternal corpulence and the concept of soft-tissue dystocia.