The rate of caesarean section is higher among obese pregnant women, leading to increased morbidity in this already vulnerable population. Oxytocin is the main drug used in obstetrics to optimize progress of labour, but observational studies have suggested that its efficiency may be insufficient in obese women with usual doses.

We design a randomised controlled trial to test the effect of an increased oxytocin dose on the rate of caesarean section in obese primiparous women with spontaneous or induced labour.

The hypothesis underlying this trial is that an increase in oxytocin dose can reduce the rate of caesarean sections in primiparous obese patients, with a spontaneous or induced onset of labour, without increasing maternal or neonatal morbidity. This would be a major step forward in reducing morbidity in an at-risk population and in improving the obstetric prognosis for future pregnancies.

The research is a double-blind controlled trial, including primiparous obese women in spontaneous or induced labour, for whom a prescription of oxytocin is decided. Oxytocin is currently indicated for notably "insufficiency of uterine contractions, at the beginning or during labour".The recommended dosage in the market authorization will be used for the control group.

The control group will receive oxytocin at 2 milli-International unit /mL and the intervention group at 4 milli-International unit /mL, controlled by pump (final volume = 500 mL) or electrical syringe (final volume = 50 mL).

The primary objective is to compare the effect of higher doses of oxytocin (intervention group) vs standard doses of oxytocin (control group) on the rate of caesarean sections in obese patients with spontaneous or induced onset of labour.

The secondary objectives will be to compare the effect of higher doses of oxytocin (intervention group) vs standard doses of oxytocin (control group) on maternal and labour complications (length of labour, arrest of labour, interruption of oxytocin perfusion and reason, uterine hyper-stimulation, mode of vaginal delivery, reason for caesarean section, post-partum haemorrhage, maternal blood transfusion, volume of oxytocin infusion, oxytocin side effects), as well as foetal complications and neonatal complications.

Inclusion criteria

Inclusion criteria are the following:

• Age ≥ 18 years
• Nulliparous (no previous childbirth beyond 22 SA)
• BMI ≥ 30 kg/m² at the beginning of pregnancy
• Singleton pregnancy
• Spontaneous or induced onset of labour
• Cephalic presentation
• Term ≥ 37 weeks of gestation and < 42 weeks of gestation
• Medical Indication and absence of medical contraindication for oxytocin during labour: inadequate and/or ineffective uterine contraction and/or delayed cervical dilation
• Written consent
• Affiliation to a french social security system

Exclusion criteria

Exclusion criteria are the following:

• Hypersensitivity to the active substance (oxytocin), to any of its excipients or to latex (risk of cross allergy)
• Medical contraindication for oxytocin
• Coagulation disorders
• Foetal growth restriction (inferior to 5th percentile)
• Foetal malformation (major)
• Foetal heart rate anomalies before use of oxytocin (at the time of inclusion)
• History of uterine surgery (scarred uterus of gynaecological origin)
• Patient with a disease requiring caesarean section prior to labour (planned caesarean section before labour)
• Severe renal failure
• Patient deprived of their liberty (under curatorship or guardianship)
• Participation in another interventional trial of category 1. Patients included in interventional research with minimal risk and constraints may participate in the study after the investigator's assessment.

pregnant women

An "out-of-protocol" healthcare professional (midwife / nurse) will prepare oxytocin. The "out-of-protocol" healthcare professional (midwife / nurse) will not be a study's investigator, and will not be involved in the patient's care. The randomization group will not be communicated to the investigator or any medical personnel involved in the study or patient by the "out-of-protocol" nurse/midwife, in order to preserve the double-blind design.