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Poster Session 2

(592) Fetal head circumference underestimation of neonatal head circumference: Results from a prospective multicenter study

Thursday, January 30, 2025
4:00 PM – 6:00 PM  
Objective:

Being able to prospectively predict difficult labor, and the need for operative vaginal delivery or cesarean delivery, is of great value in contemporary obstetric practice, in particular for nulliparous patients. Increasing fetal head circumference (FHC) has been the focus of predictors in this regard, but this assumes that prenatal ultrasound assessment of FHC is an accurate surrogate marker for neonatal head circumference (NHC). 

Study Design: We evaluated the accuracy of prenatal FHC measurement for predicting NHC in a secondary analysis of a prospective multicenter study of nulliparous term labor. 2,336 pregnancies with an ultrasound examination at 39 weeks’ gestation were included, in which FHC was measured in triplicate and averaged. NHC was measured at birth by a standard Holtain measuring tape, placed 2cm above the ears around the occipito-frontal circumference. Linear regression was used to compare differences in FHC and NHC, adjusting for time to delivery (TTD) in days. An alternative approach without adjustment for TTD, FHC and NHC were compared using the 90th centiles from the INTERGROWTH-21st study for FHC and NHC.

Results: The median [IQR] gestational age at ultrasound examination was 39.3 weeks [39.0 - 39.6], and FHC was 337mm [329-345]. The median [IQR] gestational age at delivery was 40.7 weeks [40.3 – 41.3], and NHC was 350mm [340-360]. Regression analysis indicated that FHC underestimated NHC by 10.5mm on average (difference=-10.5 – 0.37 * TTD, see Figure 1). Centile-based comparisons found that 17% of FHC measurements were above the 90th centile, compared to 30% of NHC values above the 90th centile.

Conclusion: Late pregnancy ultrasound measurement of FHC underestimates neonatal head circumference by approximately 1cm at term, and a centile-based comparison did not compensate for this difference. This systematic error may have important implications and should be taken into account in the interpretation of fetal head circumference.