To evaluate associations between early-pregnancy body mass index (BMI) and active first stage labour duration, accounting for possible interaction with maternal age, we conducted a cohort study of women with spontaneous onset of labour allocated to Robson group 1. Quantile regression analysis was performed to estimate first stage labour duration between BMI categories in two maternal age subgroups (more and less than 30 years). Results show that obesity (BMI > 30) among younger women (< 30 years) increased the median labour duration of first stage by 30 min compared with normal weight women (BMI < 25), and time difference estimated at the 90th quantile was more than 1 h. Active first stage labour time differences between obese and normal weight women was modified by maternal age. In conclusion: (a) obesity is associated with longer duration of first stage of labour, and (b) maternal age is an effect modifier for this association. This novel finding of an effect modification between BMI and maternal age contributes to the body of evidence that supports a more individualized approach when describing labour duration.
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