Re-evaluation of modifiable risk factors for obstetric anal sphincter injury in a real-world setting

The objective was to identify modifiable risk factors for obstetric anal sphincter injury (OASI) in primiparous women.

This was a retrospective cohort study of primiparous women with a singleton vaginal delivery. Main outcome measures were incidence of OASI and odds ratios for possible risk factors: maternal age, body mass index and height, fetal birthweight and head circumference, gestational age, epidural analgesia, mediolateral episiotomy, and instrumental deliveries. Univariate and multivariate logistic regressions were performed using forward methods for variable selection.

Of 19,786 primiparous women with a singleton vaginal delivery, 369 sustained an OASI (1.9%). Risk factors were identified: vacuum extraction (adjusted OR 2.06, 95% CI, 1.59-2.65, p < 0.001), increased fetal weight (aOR 1.06, 95% CI, 1.02-1.11, p = 0.002, per 100-g increments); head circumference (aOR 1.24, 95% CI, 1.13-1.35, p < 0.001, per 1-cm increments); gestational week (aOR 1.11, 95% CI, 1.02-1.2, p = 0.012, per week). Protective factors: mediolateral episiotomy (aOR 0.75, 95% CI, 0.59-0.94, p = 0.013) particularly in vacuum deliveries (aOR 0.50, 95% CI, 0.29-0.97, p = 0.040); epidural analgesia (aOR 0.64, 95% CI, 0.48-0.84, p = 0.001); maternal height ≥157 cm (aOR 0.97, 95% CI, 0.96-0.98, p = 0.006, risk decreases by 2.6% per 1 cm increase in height).

Mediolateral episiotomy was protective against OASI in both spontaneous and instrumental deliveries of primiparae. Increased fetal weight and large fetal head circumference, particularly in short women, were significant risk factors. These findings support the performance of ultrasound to acquire updated fetal measures before admission to the labor ward.