Risk of Recurrence and Subsequent Delivery After Obstetric Anal Sphincter Injuries

Population-based cohort studies have reported that obstetric anal sphincter injuries (OASIS) tend to recur in subsequent births. Few data are available on possible risk factors for recurrence. Moreover, little is known about paternal influence on the likelihood of recurrence.This population-based study was designed to investigate the likelihood of having OASIS in a subsequent pregnancy among women with a history of OASIS and to also study the effect of instrumental delivery, paternal influence, and size of maternity unit on recurrence. Data were obtained from the Medical Birth Registry of Norway for a cohort of 828,864 mothers with singleton, vertex-presenting infants, weighing 500 g or more, and delivered between 1967 and 2004. The occurrence of OASIS, subsequent delivery rate, and planned cesarean rate were compared in 2 groupswomen with and without a history of OASIS. The primary study outcome measures were recurrence of OASIS in second and third deliveries, subsequent delivery rate, and mode of delivery after third- to fourth-degree injuries.Compared with women without a prior history of OASIS, there was an increased risk of recurrence of OASIS in a subsequent delivery among women with a previous history of OASIS in the first and in both the first and second deliveries; the adjusted odds ratios were 4.2 with a 95% confidence interval (CI) of 3.9 to 4.5 and 10.6 with a 95% CI of 6.2 to 18.1, respectively, as compared with those women without prior OASIS. There was an association between risk of recurrence and instrumental deliveries (especially, forceps deliveries), birth weights >3500 g, and maternity units with >3000 deliveries per year. A man who fathered a birth child resulting in OASIS was more likely to father a subsequent birth that resulted in OASIS in another woman who gave birth; the odds ratio was 2.1, with a 95% CI of 1.2 to 3.7.These findings show that a history of OASIS is associated with a high risk of recurrence in second and third deliveries. Additionally, women with a prior OASIS have planned cesarean delivery more frequently.