Fetal head prior to delivery: precise measurement, using a mathematically extrapolated frontooccipital diameter

Purpose To find a method that could improve the accuracy of fHC evaluation at term. Material and methods This retrospective cohort study was based on data from the University Hospital of Zürich (USZ). Study population included singleton fetuses and newborns at term between 2015 and 2017. Fetal biometry was performed within 1 week prior to delivery. Study data consisted of two cohorts with 200 ultrasound measurements each: 200 performed by an expert, 200 by residents. fHC were compared with the newborn’s head circumference (nHC). fHC was estimated using two methods: (1) fHC based on sonographically estimated frontooccipital diameter (FOD) and biparietal diameter (BPD). (2) Expected FOD was calculated as a fixed mean FOD/BPD relation, derived from biometry standards as the 50th percentile. If BPD was < 99 mm, FOD was calculated according to the formula BPD × 1.268557, If BPD was ≥ 99 mm, FOD was calculated according to the formula BPD × 1.20641443. Results fHC was underestimated compared with nHC in the expert group: percentage error (PE =  − 3.68%, SD = 2.79), as well as in the group of residents (PE =  − 3.78%, SD = 3.20) using method 1. fHC measurement was significantly more accurate using method 2. In the expert group PE =  − 1.17%, SD = 3.08, in the group of residents PE =  − 0.95%, SD = 3.33. Bland–Altman analysis showed limits of agreement (LOA) between − 2.41338 and 0.5537828 cm for fHC. Conclusions Accuracy of fHC estimation could be improved by extrapolation of FOD when real-time FOD cannot be measured