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.