Put your weight behind it – effect of BMI 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 prepregnant 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 foetus 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 Kaplan-Meier analyses.
We performed stratified analyses in women with and without epidural
analgesia and oxytocin augmentation. Finally, we calculated the Hazard
Ratio (HR) of delivery using Cox regression 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: The final study population comprised
23516 women in TGCS group 1 and 27255 in group 3. Increasing BMI was
associated with shorter estimated median duration of the active phase in
both TGCS groups. The pattern remained when performing stratified
analyses for epidural and oxytocin analgesia. The HR for delivery
increased significantly with increasing BMI analysed as a continuous
variable; HR 1,019 (95%CI 1.016-1.023) in TGCS group 1 and HR 1,017
(95%CI 1.014-1.020) in TGCS group 3. Conclusion: Increasing BMI was
associated with shorter estimated median duration of the active second
stage Funding: None.