AbstractIntroductionThe aim of the study was to compare the duration of active phase of labor in women with spontaneous or induced start of labor.Material and methodsAn observational cohort study was performed at Stavanger University Hospital in Norway between January 2010 and December 2013. During the study period 19 524 women delivered. Data for the study were collected from an electronic birth journal. Women with previous cesarean section, multiple pregnancy, breech or transverse lie, preterm labor or prelabor cesarean section were excluded. Analyses were stratified between nulliparous and parous women. Active phase of labor was defined when contractions were regular, with cervix effaced and dilated 4 cm. The main outcome measure was duration of active phase of labor.ResultsThe active phase was longer in induced labors than in labors with spontaneous onset in nulliparous women. The estimated median duration using survival analyses was 433 min (95% confidence interval 419–446) in spontaneous vs. 541 min (95% confidence interval 502–580) in induced labors [unadjusted hazard ratio 0.76 (95% confidence interval 0.71–0.82) and adjusted hazard ratio 0.88 (95% confidence interval 0.82–0.95)]. In parous women, a one minus survival plot showed that induced labors had shorter duration before six hours in active labor, but after six hours, induced labors had longer duration. The overall difference in parous women was small and probably of little clinical importance.ConclusionThe active phase of labor was longer in induced than in spontaneous labors in nulliparous women.