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Background  Injury to the genital tract sustained during childbirth can lead to transient or protracted morbidity. Attention should be paid to avoidable risk factors that can cause this complication.

Aim  To analyse the recurrence, at a later delivery, of trauma to the genital tract, subsequent to perineal laceration of the sphincter ani (third or fourth degree), sustained at an earlier delivery.

Design  A population-based study.

Setting  In Sweden, 1973–1997 inclusive.

Population  All women with a vaginal, singleton delivery in Sweden.

Methods  The Medical Birth Registry, the National Board of Health and Welfare was used to identify cases of ruptured sphincter ani.

Main outcome measures  OR was calculated with 95% confidence interval. A stratified analysis was performed using the Mantel-Haenszel technique.

Major end point  Rupture of the sphincter ani (third or fourth degree) at second delivery.

Results  The incidence of anal sphincter rupture increased sixfold during the study period, from 0.5% in 1973 to 3.0% in 1997. Women who had sustained a laceration of this type ran a significantly increased risk of a recurrence at a later delivery. This effect persisted even after stratification for birthweight, year of birth, parity and maternal age (OR 4.74, 95% confidence interval 4.34–5.17). When only fourth degree rupture was considered (rupture of both anal sphincter and rectum), the corresponding figures were 6.52 (95% CI 5.29–8.04). This effect also persisted after stratification for birthweight, year of birth, parity and maternal age. The OR for giving birth a second time, subsequent to a third or fourth degree perineal laceration at first delivery, was 0.68 (95% CI 0.67–0.70).

Conclusion  Our findings suggest that the risk of an anal sphincter rupture at delivery increases five to sevenfold when there has been a similar rupture at a previous delivery. Further study is needed before safe recommendations can be made concerning the subsequent mode of delivery to be adopted, following rupture in the sphincter ani at a previous birth.