Agreement Between WHO and Thai Fetal Growth Charts for Classifying Estimated Fetal Weight in Thai Singleton Pregnancies: A Retrospective Cross-Sectional Study.

BACKGROUND AND AIMS: Universal fetal growth standards may classify fetal size differently from locally derived charts, particularly in populations with different anthropometric characteristics. This study aimed to compare the agreement and classification patterns of the WHO and Thai fetal growth charts when applied to the same ultrasound-derived estimated fetal weight (EFW) values in a Thai clinical cohort. METHODS: This retrospective analytical study included 1109 Thai pregnant women between 21 and 42 weeks of gestation who received antenatal care at the Faculty of Medicine, Chiang Mai University. Ultrasound biometric parameters-biparietal diameter, head circumference, abdominal circumference, and femur length-were used to estimate fetal weight via the Hadlock-3 formula. EFW percentiles were calculated using both the WHO fetal growth standards and the Thai fetal growth chart. The proportions of fetuses classified as SGA (< 10th percentile) and LGA (> 90th percentile) were compared. RESULTS: The WHO chart classified a significantly higher proportion of fetuses as SGA (27.1%) compared to the Thai chart (12.7%) (p < 0.001), while identifying fewer fetuses as LGA (3.3% vs. 5.4%, p < 0.001). Although the percentile rankings from the two charts were significantly correlated (Spearman's correlation coefficient of 0.750, p < 0.001), the correlation was moderate. Bland-Altman analysis indicated relatively poor agreement between the two methods, with discrepancies that may be clinically relevant. CONCLUSION: The WHO and Thai fetal growth charts produced different fetal size classifications in this cohort, with the WHO chart classifying more SGA and fewer LGA fetuses, particularly in late gestation. Further outcome-based validation is needed to determine the clinical performance of each chart.