Use of gestation adjusted projection method for predicting fetal head circumference.

Objectives: The aim of this study is to determine if the gestation adjusted projection (GAP) method applied to a fetal head circumference (FHC) measured on ultrasound between 32 and 36 weeks and 6 days gestation can predict birth head circumference, specifically ≥ 35 cm, which is a known risk factor for Cesarean.

Methods: This is a retrospective chart review of 60 pregnancies from January to December 2019. Eligible patients delivered a singleton term neonate and received two ultrasounds, one at 32-36 weeks and 6 days gestation (period 1) and a second within 7 days of a term birth (period 2). Fetal head circumference was predicted two ways, by applying (1) the GAP method to the period 1 ultrasound and (2) by direct measurement with a period 2 ultrasound. These estimates were compared to the birth head circumference (HC_BIRTH) by measures of error and with paired t-tests. McNemar's test compared the ability to predict head circumference (HC) ≥ 35 cm.

Results: None of the measures of error were significantly different between the GAP and the period 2 ultrasound, including the ability to predict HC ≥ 35 cm. In patients who delivered at ≥ 39 weeks, the period 2 ultrasound performed poorly while the GAP's performance remained good.

Conclusion: The GAP method applied to an early third trimester ultrasound predicts HC_BIRTH with accuracy similar to an ultrasound performed seven days from delivery and may be superior for deliveries ≥ 39 weeks. The ability to predict HC_BIRTH could improve clinical management of affected pregnancies.