Can sonographic fetal head circumference measurement help predict obstetric anal sphincter injury?

Objective: To investigate whether sonographic fetal head circumference (sfHC) size assessed by ultrasound prenatally could help predict OASIS.

Methods: This study is a retrospective cohort study between 2005 and 2016. Antenatal sfHC assessment, maternal demographics, and labor and delivery data were abstracted and compared in women with and without OASIS.

Results: Of 2057 pregnant women that had sfHC assessment, 121 (5.8%) had OASIS. In the unadjusted analysis, sfHC above the 90th centile was associated with OASIS (odds ratio [OR] 1.12; 95% confidence interval [CI] 1.02-1.23; P = 0.015). When adjusted for gestational age at delivery, maternal age, race, prolonged second stage, body mass index, infant gender, and intrapartum oxytocin use, sfHC above the 90th centile remained significantly associated with OASIS (OR 1.13, 95% CI 1.00-1.27, P = 0.050).

Conclusion: In our cohort, sfHC above the 90th centile was associated with a greater risk of OASIS. As OASIS significantly impacts both short-term and long-term health outcomes, such as perineal pain, dyspareunia, and urinary and fecal incontinence, sfHC could be an additional prenatal marker to help clinicians counsel pregnant women about the risk of OASIS.