Put your weight behind it—Effect of body mass index on the active second stage of labour: A retrospective cohort study

OBJECTIVE: To explore the duration of the active phase of the second stage of labour in relation to maternal pre‐pregnant body mass index (BMI). DESIGN: Retrospective cohort study. SETTING: Labour wards of three Norwegian university hospitals, 2012–2019. POPULATION: Nulliparous and parous women without previous caesarean section with a live singleton fetus in cephalic presentation and spontaneous onset of labour, corresponding to the Ten Group Classification System (TGCS) group 1 and 3. METHODS: Women were stratified to BMI groups according to WHO classification, and estimated median duration of the active phase of the second stage of labour was calculated using survival analyses. Caesarean sections and operative vaginal deliveries during the active phase were censored. MAIN OUTCOME MEASURES: Estimated median duration of the active phase of second stage of labour. RESULTS: In all, 47 942 women were included in the survival analyses. Increasing BMI was associated with shorter estimated median duration of the active second stage in both TGCS groups. In TGCS group 1, the estimated median durations (interquartile range) were 44 (26–75), 43 (25–71), 39 (22–70), 33 (18–63), 34 (19–54) and 29 (16–56) minutes in BMI groups 1–6, respectively. In TGCS group 3, the corresponding values were 11 (6–19), 10 (6–17), 10 (6–16), 9 (5–15), 8 (5–13) and 7 (4–11) minutes. Increasing BMI remained associated with shorter estimated median duration in analyses stratified by oxytocin augmentation and epidural analgesia. CONCLUSION: Increasing BMI was associated with shorter estimated median duration of the active second stage of labour.