Background:
Obstetric anal sphincter injury (OASI), in which major tearing of the perineum during childbirth extends into the anal sphincter, has  consequences for women. For a woman having a subsequent birth, major damage can occur again, resulting in an OASI recurrence. A history of an OASI thus raises clinical questions regarding mode of delivery and potential risk for subsequent births.  To date, studies investigating factors associated with recurrence have not assessed the role of first birth factors.

Aim:
To determine the OASI recurrence rates at the second birth for women who have previously sustained an OASI at their first birth; to identify the modes of delivery for the second birth; and to identify significant predictive factors for recurrence from both the first and second births.

Methods:
Data were obtained from NSW population-based collections – the Perinatal Data Collection and the Admitted Patients Data Collection. These were linked using probabilistic methods. The study population consisted of women with at least a first and second consecutive birth during 2001-2011, where the second was term  and vertex. Predictive factors for recurrence were determined using multivariable logistic regression.

Results:
Among women with a first birth OASI, 22% had a caesarean section for their second birth. Among second vaginal births, the OASI recurrence rate was 5.8%, with risk factors from the first birth identified including birthweight < 3kg (aOR 1.72); and from the second birth forceps delivery without episiotomy (aOR 5.95). Protective factors from the first birth included regional analgesia (aOR 0.72), and from the second, oxytocin (aOR 0.61).

Conclusion:
An OASI recurs for approximately one in twenty women who have a second vaginal birth. Knowledge of the risk factors for an OASI recurrence can provide information to clinicians and women to help inform decisions around mode of subsequent delivery.