Sonographic fetal head circumference and the risk of obstetric anal sphincter injury following vaginal delivery

Introduction: High birth weight is strongly associated with OASIS; nevertheless, it has not been determined which biometric characteristics most affect OASIS occurrence. We aimed to evaluate the association of estimated fetal head circumference with OASIS occurrence among primiparous women delivering by unassisted vaginal delivery. Methods: A retrospective study included all primiparous women who delivered at term by spontaneous vaginal delivery from 2011–2019. Women were allocated to two groups: (1) those who experienced OASIS and (2) those who did not experience OASIS. Risk factors for OASIS were analyzed. Results: Overall, 7646 women were included in the study cohort. Of those, 119/7646 (1.6%; 95% CI, 1.3–1.9%) experienced OASIS. Sonographic head circumference and birth weight did not vary between groups. Prolonged second stage was more common in the OASIS group [23 (19%) vs. 986 (13.3%), 1.58 OR (95% CI 1.003–2.51, p = 0.04)]. Absence of epidural analgesia was more common in the OASIS group [30 (25%) vs. 1197 (15.9%), 1.8 OR (95% CI 1.1–2.7, p = 0.006)]. On multivariate logistic regression analysis, the lack of epidural analgesia and duration of second stage of labor were both independently positively associated with OASIS [adjusted OR 2.67 (95% CI 1.55–4.62), p < 0.001, adjusted OR 1.23 (95% CI 1.11–1.43), p < 0.001, respectively)]. Conclusion: Sonographic head circumference and birth weight are not associated with OASIS occurrence among primiparous women delivering by an unassisted vaginal delivery. Prolonged second stage and the use of epidural analgesia are modifiable risk factors among these women.