Consolidated and updated ultrasonographic fetal biometry and estimated fetal weight references for the Hong Kong Chinese population.

Introduction: This study aimed to construct consolidated and updated ultrasonographic fetal biometry and estimated fetal weight (EFW) references for the Hong Kong Chinese population and evaluate the extent of under- and overdiagnosis of small-for-gestational-age (SGA) and large-for-gestational-age (LGA) using these new references.

Methods: Fetal biometry and EFW references were constructed using the Generalised Additive Model for Location, Scale, and Shape, based on data from 1679 singleton pregnancies in non-smoking Chinese women. Ultrasound scans were performed at 12 to 40 weeks of gestation to measure biparietal diameter, head circumference, abdominal circumference (AC), and femur length, following standardised protocols. The rates of SGA and LGA diagnoses using the existing and updated Hong Kong fetal biometry references were compared in an independent cohort of 10 229 pregnancies.

Results: The median number of scans per gestational week between 20 and 39 weeks was 75 (interquartile range=67-83). Compared with existing references, the new AC reference would significantly (P<0.001) increase the proportions of SGA fetuses with AC measurements at <3rd and <10th percentiles from 1.7% and 6.1% to 3.4% and 10.0%, respectively. Conversely, it would significantly decrease (P<0.001) the proportions of LGA fetuses with AC at >90th and >97th percentiles from 15.0% and 4.9% to 11.5% and 3.5%, respectively.

Conclusion: Adoption of the new references, particularly for AC, may lead to increased identification of SGA cases and decreased identification of LGA cases. The proportions of these cases will be more consistent with their intended diagnostic thresholds. Further studies are needed to determine how these references impact pregnancy outcomes.