The role of fetal head circumference in the formation of obstetric anal sphincter injuries following vacuum deliveries among primiparous women

Purpose: To evaluate the association of sonographic fetal head circumference (HC) with obstetric anal sphincter injury (OASIS) occurrence among primiparous women who underwent vacuum-assisted delivery (VAD). Methods: A retrospective study of all primiparous women who delivered at term by VAD between 2011 and 2019 and underwent ultrasound with fetal biometry within 1 week prior to delivery. Women who suffered OASIS were compared to women without OASIS. Results: Overall, 74 of 3222 (2.3%) primiparous women suffered an OASIS. As compared with control, women with OASIS were younger (median 28 vs. 30 years, p = 0.001), had higher BMI (median 28.2 vs. 26.9 kg/m², p = 0.03), and had a longer second stage of labor (median 190 vs. 168 min, p = 0.01). Fetal head circumference was larger in the OASIS group (mean 334 vs. 330 mm, p = 0.03), occiput posterior fetal head position was more prevalent (12 (16%) vs. 232 (7.4%), OR [95% CI]: 2.43 (1.29–4.57), p = 0.004), and the rate of mediolateral episiotomy performed was lower (58 (78.0%) vs. 2777 (88.2%), OR [95% CI]: 0.48 (0.27–0.85), p = 0.01). Multivariate regression modeling identified higher fetal HC (aOR [95% CI] 1.03 (1.001–1.06), p = 0.04) and occiput posterior (aOR [95% CI] 2.5 (1.16–5.71), p = 0.01) as independently positively associated with OASIS. Mediolateral episiotomy and maternal age were independently negatively associated with an OASIS (aOR [95% CI] 0.39 (0.18–0.85), p = 0.01); aOR [95% CI] 0.4 (0.17–0.60), p = 0.001). Conclusions: Sonographic large fetal HC is associated with OASIS occurrence during VAD. The only modifiable predictor of OASIS detected was mediolateral episiotomy, found to be protective against OASIS.