Participants
At Innlandet Hospital Trust, Lillehammer, Norway, detailed information on maternal health, pregnancy, delivery and the postpartum period until discharge is prospectively registered in a perinatal database. This hospital covers virtually all births in a region with a population of around 90,000 people at the time of the study; around 23,000 lived in the city Lillehammer and the others in rural areas with small towns. The women were generally first registered in the perinatal database at 18-20 weeks’ gestation when they met for the routine ultrasound assessment. This study included all deliveries that occurred during January 1st 1990 to December 31st 2002. We identified singleton vaginal deliveries with gestational age >210 days where women for the first time were diagnosed with perineal rupture from the database, and the data were quality assured and expanded by scrutinizing delivery protocols, charts, and patient records. Women with 3rd and 4th degree OASI were defined as cases, and we selected the next vaginal singleton delivery with the same parity and gestational age >210 days without OASI as a matched control.