Modifiable and non-modifiable risk factors for obstetric anal sphincter injury in a Norwegian Region: A case-control study.

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.