OBJECTIVE: To analyze circumstances relating to severe anal sphincter tears occurring at spontaneous delivery, in view of reported differences in practice regarding manual perineal protection during delivery. DESIGN: Cohort study of midwife-conducted non-operative vaginal deliveries. SETTING: Five Norwegian hospitals with 12,438 consecutive deliveries during a 12-month period. METHODS: Data from 357 women sustaining third and fourth grade anal sphincter tears (2.9%) were analyzed. MAIN OUTCOME MEASURES: Different incidence of major perineal tears. RESULTS: Sphincter tear incidence varied significantly between the five hospitals, from 1.3 to 4.7% (p