Can Fetal Head Circumference Assessment on Ultrasound Help Predict 3rd and 4th Degree Perineal lacerations? [40M]

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Obstetric Anal Sphincter Injuries (OASIS) occur in 2 to 19% of vaginal deliveries in the United States. Neonatal measurements such as birthweight above 4000g or neonatal head circumference above 37cm are known risk factors for 3rd and 4th degree perineal laceration. Unfortunately, these predictors are identified after the delivery and therefore have very limited practical impact. We hypothesized that fetal Head Circumference (HC) size assessed by ultrasound in pregnancy can aid in predicting Obstetric Anal Sphincter Injuries (OASIS).


Secondary analysis of a retrospective cohort study assessing pregnancies with OASIS between 2005 and 2016. Inclusion criteria for this secondary analysis were third trimester ultrasound head circumference measurements. Antenatal ultrasound Head Circumference (HC) assessment, maternal demographics and labor and delivery data were documented and compared. P less than .05 was considered statistically significant.


Of 2057 patients that had head circumference assessment, 121 (5.8%) had OASIS. In the unadjusted continuous analysis of the head circumference, an increase of 10% in the HC were associated with OASIS OR 1.12 (1.02-1.23) p=0.015. Although not statistically significant HC above the 90% appears to be associated with OASIS OR 3.37 (0.89-12.74) p=0.073.


In our cohort, fetal head circumference percentile is associated with OASIS, specifically the larger the head circumference percentile, the greater the risk. Severe perineal lacerations significantly impact short and long term health outcomes such as perineal pain, dyspareunia, urinary and fecal incontinence. This piece of information might help clinicians antenatally during counseling of patients about the risk of OASIS and future implications of the injury.

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