Objective: To quantify the effects of pre-pregnancy body mass and gestational weight gain, above and beyond their known effects on birthweight, on the risk of primary and repeat caesarean delivery performed before or after the onset of labour.
Design: Hospital-based historical cohort study.
Setting: Canadian university-affiliated hospital.
Population: A total of 63 390 singleton term (> or = 37 weeks gestation) infants with cephalic presentation.
Methods: We studied prospectively archived deliveries at the Royal Victoria Hospital in Montreal, Canada, from 1 January 1978 to 31 March 2001 using multiple logistic regression models to estimate relative odds of caesarean delivery.
Main outcome measure: Caesarean delivery, primary or repeat and before or after the onset of labour.
Results: Pregravid obesity (body mass index > or = 30 kg/m2) increased the likelihood of primary caesarean delivery before (OR = 2.01, 95% CI 1.39-2.90) and after (OR = 2.12, 95% CI 1.86-2.42) the onset of labour. High net rate of gestational weight gain (> 0.50 kg/week) increased the risk but only after labour onset (OR = 1.40, 95% CI 1.23-1.60). Among women with a previous caesarean, high weight gain modestly increased risk but only before labour (OR = 1.38, 95% CI 1.04-1.83), whereas obesity increased the risk of caesarean delivery both before (OR = 1.85, 95% CI 1.44-2.37) and after (OR = 1.96, 95% CI 1.11-3.47) labour onset. Increased risks of macrosomia accounted for the association between pregravid adiposity and repeat caesarean delivery performed after but not before the onset of labour.
Conclusions: Pregravid obesity increases the risk of caesarean delivery both before and after the onset of labour and both with and without a history of caesarean.