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/m, 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.