Nulliparous women in active labor at the selected study locations were invited to take part in the study. Inclusion criteria were: a singleton pregnancy, gestational age between 37–42 weeks, no maternal /fetal chronic and/or pregnancy-related conditions. A spontaneous onset of labor, regular contractions and cervical dilation of at least 3 cm were required before the women were invited to take part in the study. A group of midwives and medical doctors at each clinic assisted with data collection and with enrolling women into the study. For all included deliveries a partogram was recorded to monitor the progress of labor. Included clinics used different alert lines of the partogram, but two criteria for labor dystocia were defined: if cervical dilation crossed the action line in the partogram or if labor progress was arrested for two hours or more. Oxytocin was administered according to local clinical guidelines, but the amount of oxytocin added to the infusion did not differ between clinics. A flowchart of the study design is presented in Fig 3.