Background
There is no consensus to the implications of an increased sonographic fetal head circumference (HC) and its impact on delivery.

Aim
To examine if there is any association between sonographic fetal HC, obstetric anal sphincter injury (OASIS) and mode of delivery.

Materials and Methods
A retrospective cohort study of term, singleton births between April 2017 and March 2019 at a large regional hospital in Australia with a third trimester ultrasound. Logistic regressions were performed investigating sonographic fetal HC and additional risk factors for OASIS. Further multinomial logistic regressions assessed the relationship between the sonographic HC and mode of delivery. Odds ratios and their 95% CIs were reported.

Results
Of 667 eligible women, 487 (73%) had vaginal births, with 32 (6.6%) sustaining an OASIS and 180 (27%) had caesarean sections (CS). The sonographic fetal HC did not show an association with OASIS (odds ratio 1.005; CI 0.99–1.01, P = 0.447). A statistically significant association (P < 0.05) with OASIS was found with Asian ethnicity (4.38; 1.5–11.32), prolonged second stage (≥2 h) (4.26; 1.57–10.49) and occiput posterior position (4.01; 1.08–11.92). For women with a sonographic fetal HC ≥ 90th percentile, the odds of having CS compared to a spontaneous vaginal birth are 2.77 (95% CI: 1.36, 5.62; P = 0.005) times higher than those who have a HC < 90th percentile. Conclusion This study does not support the use of sonographic fetal HC in assessing a woman’s risk of sustaining an OASIS. Sonographic fetal HC is associated with mode of delivery.