Inclusion Criteria:  nulliparous women singleton pregnancy social insurance uncomplicated pregnancy no contraindication to vaginal delivery  Exclusion Criteria:  scheduled caesarean section labor < 37 weeks of gestational age cervical dilation > 8cm at inclusion scheduled induced delivery contraindication to pushing effort during labor and delivery BMI > 40 kg/m2 medical history of daibetes mellitus, hypertension, heart disease