Pelvic dimensions and hypotheses on duration of active second stage of labourTilde Broach OstborgStavanger University HospitalTM EggeboTrondheium University HospitalWe would like to thank Jan Novák and Petr Sedlak for their interest and comments to our manuscript. We found that increasing BMI was associated with shorter estimated median duration of the active second stage of labour.1We could not find any obvious causal mechanism for our findings; but suggested some possible explanations. The shorter active second stage may be related to increased abdominal pressure with increasing BMI, or perhaps increased strength when pushing.2, 3 Increased infiltration of fat in the muscular pelvic floor may decrease its strength and resistance.4 The presence of fat in the birth canal of obese women may delay the urge to bear down, thereby postponing active pushing until the head is lower in the maternal pelvis.Novak et al. measured the bi-ilac and bi-cristal diameters of the greater pelvis and found a broader pelvis in individuals with a history of obesity from adolescence.5 We supposed that there would be an association between the size of the greater pelvis and the size of the birth canal. We agree to the limitations commented by Novák and Sedlak. However, our proposed causal mechanisms are merely hypotheses, and cannot be accepted nor rejected based on current knowledge.1. Ostborg TB, Sande RK, Kessler J, Tappert C, von Brandis P, Eggebo TM. Put your weight behind it-Effect of body mass index on the active second stage of labour: A retrospective cohort study. BJOG. 2022;129:2166-2174.2. Lambert DM, Marceau S, Forse RA. Intra-abdominal pressure in the morbidly obese. Obes Surg. 2005;15:1225-1232.3. Tomlinson DJ, Erskine RM, Morse CI, Winwood K, Onambele-Pearson G. The impact of obesity on skeletal muscle strength and structure through adolescence to old age. Biogerontology. 2016;17:467-483.4. Pomian A, Lisik W, Kosieradzki M, Barcz E. Obesity and Pelvic Floor Disorders: A Review of the Literature. Med Sci Monit. 2016;22:1880-1886.5. Novak JM, Bruzek J, Zamrazilova H, Vankova M, Hill M, Sedlak P. The relationship between adolescent obesity and pelvis dimensions in adulthood: a retrospective longitudinal study. PeerJ. 2020;8:e8951.

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.