Objective: To investigate the impact of severe obesity (body mass index [BMI] ≥35 kg/ m 2 ) on uterine contractile activity. The hypothesis was that obese parturients might have weaker uterine activity and need more oxytocin than leaner parturients.Design: Exploratory, blinded analysis of a randomised controlled trial cohort.Setting: Two labour wards, one in a university tertiary hospital and one in a central hospital. Population: In all, 686 parturients with singleton pregnancies, gestational age ≥37 weeks, fetus in cephalic presentation, and intrauterine tocodynamometry during labour. [Correction added on 6 June 2022, after first online publication: the number of parturients has been corrected to 686.] Methods: Uterine contractile activity was assessed as intrauterine pressure, frequency of contractions and basal tonus of uterine muscle. The use of oxytocin and cervical dilatation were recorded simultaneously. Main outcome measures: Primary outcome: uterine contractile activity. Secondary outcomes: use of oxytocin, labour outcomes.Results: Obese parturients reached intrauterine pressure ≥200 Montevideo units (MVU) during the first stage of labour more often than leaner parturients; 62% vs 49%; odds ratio [OR] 1.67 (95% CI 1.05-2.67) and had higher basal tone of uterine muscle. However, obese parturients without previous vaginal delivery were not able to reach the active stage of labour as often as leaner ones, and their vaginal delivery success rate was lower. If a parturient had had previous vaginal delivery, obesity did not influence uterine activity, nor was there a risk of caesarean section. Doses and total consumption of oxytocin did not differ between BMI groups.Conclusions: Obese nulliparas have stronger uterine contractile activity than leaner ones, but they more often fail to reach the active phase of labour and their vaginal delivery success rate is lower.

(BJOG. 2022;129:1790–1797) With conflicting results of previous studies on the need for increased oxytocin during labor for obese parturients, there is no clear consensus on the causes of halted labor progression in this population. Studies of intrauterine pressure (IUP) in obese women during labor are also contradictory, and some show that even with satisfactory IUP [≥200 Montevideo units (MVU)] there is still risk of slowed labor progression and unplanned cesarean delivery (CD). This study aimed to determine whether, when compared to leaner parturients, the contractions of the obese parturient are less powerful, and if greater amounts of oxytocin are needed for successful vaginal delivery (VD).