II.

The validity of a diagnosis of obstetric sphincter tears, based on the MBRN, is sufficiently high to justify future large-scale epidemiologic studies based on this database, while the validity of a PAS diagnosis is lower, but improves.

Objective To investigate the recurrence risk, the likelihood of having further deliveries and mode of delivery after third to fourth degree obstetric anal sphincter injuries (OASIS).Design Population-based cohort study.Setting The Medical Birth Registry of Norway.Population A cohort of 828 864 mothers with singleton, vertexpresenting infants, weighing 500 g or more, during the period 1967-2004.Methods Comparison of women with and without a history of OASIS with respect to the occurrence of OASIS, subsequent delivery rate and planned caesarean rate.Main outcome measures OASIS in second and third deliveries, subsequent delivery rate and mode of delivery.Results Adjusted odds ratios of the recurrence of OASIS in women with a history of OASIS in the first, and in both the first and second deliveries, were 4.2 (95% CI 3.9-4.5; 5.6%) and 10.6 (95% CI 6.2-18.1; 9.5%), respectively, relative to women without a history of OASIS. Instrumental deliveries, in particular forceps deliveries, birthweights of 3500 g or more and large maternity units were associated with a recurrence of OASIS. Instrumental delivery did not further increase the excess recurrence risk associated with high birthweight. A man who fathered a child whose delivery was complicated by OASIS was more likely to father another child whose delivery was complicated by OASIS in another woman who gave birth in the same maternity unit (adjusted OR 2.1; 95% CI 1.2-3.7; 5.6%). However, if the deliveries took place in different maternity units, the recurrence risk was not significantly increased (OR 1.3; 95% CI 0.8-2.1; 4.4%). The subsequent delivery rate was not different in women with and without previous OASIS, whereas women with a previous OASIS were more often scheduled to caesarean delivery.Conclusion Recurrence risks in second and third deliveries were high. A history of OASIS had little or no impact on the rates of subsequent deliveries. Women with previous OASIS were delivered more frequently by planned caesarean delivery.