Introduction
Most women experience perineal trauma when giving birth (1). Severe perineal lesions, referred to as obstetric anal sphincter injury (OASI), are diagnosed in as many as 11% of vaginal deliveries, but with significant variation between studies and national birth statistics (1-5). The true incidence rate for severe lesions may be in the range of 10–26% because the injuries can be overlooked at the delivery wards or be occult (4, 6, 7). Apart from the immediate perineal pain, OASI often has short- and long-term consequences including negative impact on sexual life and quality of life in general (8-12) as well as anal incontinence (11-15).
Adequate clinical examination following delivery is pivotal in the diagnosis of OASI (6, 16-19), and increased awareness and training of health care personnel have resulted in a doubling of detection rates (2, 18, 19). Alongside the focus on detection, prevention has gained increasing attention. Obstetric training programs for midwives with emphasis on potential preventive measures, such as attention to maternal birth position and perineal massage during the second stage of labour, have been suggested as ways of decreasing the risk of OASI (20-22). Implementation of a preventive program in five maternity clinics in Norway resulted in decreased prevalence of OASI (23), as has similar programs in other European countries (14, 24-27). However, the evidence of persistent efficacy of preventive programs is low, partly because the existing studies were assessed shortly after their introduction (28). In a study involving the four large Nordic countries over seven years, a lasting reduction was only observed in Norway (29).
Established risk factors for OASI include primiparity, vaginal birth after caesarean delivery, advanced maternal age, high birthweight, fetal occiput posterior presentation, induction and augmentation of labour, instrumental delivery, increased duration of second stage of labour, episiotomy, and Asian ethnicity (1, 2, 22, 30-32). A recent meta-analysis showed that the incidence of OASI remains high and it highlighted a need to search for hitherto unrecognized and potentially modifiable risk factors (1). We aimed at exploring both modifiable and non-modifiable risk factors in a large retrospective case-control study based on a regional cohort where detailed information related to maternal, pregnancy, delivery, and fetal characteristics had been collected prospectively.