Objective To identify modifiable and non-modifiable risk factors for
severe obstetric anal sphincter injury (OASI) following vaginal
delivery. Design Retrospective case-control study. Setting Single center
maternity clinic in South-Eastern Norway Population Women diagnosed with
OASI following singleton vaginal birth after 30 weeks’ gestation (n =
421) and matched controls (n = 421) during 1990-2002. Methods Data were
extracted retrospectively from an institutional birth registry. For each
woman with OASI the first subsequent vaginal singleton delivery matched
for parity was elected as control. Potential determinants for OASI were
assessed by conditional logistic regression analyses. Main outcome
measure OASI, defined as 3rd or 4th degree obstetric anal sphincter
lesions. Results Among modifiable factors amniotomy was the strongest
independent determinant for OASI in both primi- (adjusted odds ratio
[aOR] 4.84; 95% CI 2.60–9.02) and multiparous (aOR 3.76; 95% CI
1.45–9.76) women, followed by augmentation with oxytocin (primiparous:
aOR 1.63; 95% CI 1.08–2.46, multiparous: aOR 3.70; 95% CI
1.79–7.67). Vacuum extraction and forceps delivery were independently
associated with OASI in primiparous women (vacuum: aOR 1.91; 95% CI
1.03–3.57, forceps: aOR 2.37; 95% CI 1.14–4.92), and episiotomy for
OASI in multiparous women (aOR 2.64; 95% CI 1.36–5.14). Conclusions
Amniotomy may be a hitherto unrecognized independent modifiable risk
factor for OASI and should be further investigated for its potential
role in preventive strategies for OASI. Funding Innlandet Hospital Trust
research fund, grant number 150434. Keywords Obstetric anal sphincter
injury; OASI; Birth; Birth injury; Modifiable risk factor; Amniotomy.
Modifiable and non-modifiable risk factors for obstetric anal sphincter injury in a Norwegian Region: A case-control study.