Fetal head circumference and length of second stage of labor are risk factors for levator ani muscle injury, diagnosed by 3-dimensional transperineal ultrasound in primiparous women

Objective: We evaluated rate of levator ani muscle (LAM) avulsion among primiparae using 3-dimensional transperineal ultrasound to identify possible risk factors for such trauma. Study Design: We conducted a prospective observational study. Three-dimensional transperineal ultrasound was performed on all subjects. Primiparae were evaluated 24-72 hours after vaginal delivery. In all, 32 nulliparous gravidae (35-41 weeks) and 15 elective cesarean delivery primiparae were evaluated as methodological controls. We compared newborn head circumference (HC), birthweight, second stage duration, maternal age, baby sex, episiotomy, and instrumental delivery. Results: LAM trauma was observed in 39 of 210 (18.8%) patients and no controls. Odds ratio for LAM trauma when newborn HC ≥ 35.5 cm was 3.343 (95% confidence interval, 1.33-8.42); when second stage duration ≥ 110 minutes, odds ratio was 2.27 (95% confidence interval, 1.07-4.81). Logistic regression showed that HC ≥ 35.5 and second stage duration ≥ 110 minutes increased odds of LAM trauma by a factor of 5.32. Conclusion: Large HC and prolonged second stage duration are risk factors in LAM trauma. Elective cesarean delivery may prevent LAM trauma.