To investigate risk factors for obstetric anal sphincter injuries in a large population-based data set, and to assess to what extent changes in these risk factors could account for trends in obstetric anal sphincter injuries.
This is a population-based cohort study on data from the Medical Birth Registry of Norway between 1967 and 2004, including all vaginal singleton deliveries of vertex-presenting fetuses weighing 500 g or more. Women with their first birth before 1967 and births with previous obstetric anal sphincter injuries were excluded, leaving 1,673,442 births for study. The outcome variable was third- and fourth-degree obstetric anal sphincter injuries. The associations of obstetric anal sphincter injuries with possible risk factors were estimated by odds ratios (ORs) obtained by logistic regression.
The occurrence of obstetric anal sphincter injuries increased from 0.5% in 1967 to 4.1% in 2004. After adjusting for demographic and other risk factors, as well as possible confounders, the increase of obstetric anal sphincter injuries persisted, although reduced (unadjusted OR 7.1; 95% confidence interval [CI] 6.8–7.4; adjusted OR 5.6; 95% CI 5.3–5.9). Obstetric anal sphincter injuries were significantly associated with maternal age 30 years or older, vaginal birth order of one, previous cesarean delivery, instrumental delivery, episiotomy, type 1 diabetes, gestational diabetes, induction of labor by prostaglandin, size of maternity unit, birth weight 3,500 g or more, head circumference 35 cm or more, and African or Asian country of birth.
Risk of obstetric anal sphincter injuries considerably increased in Norway in 1967 to 2004. Changes in the risk factors studied could only partially explain this increase.
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