INTRODUCTIONWith the global outbreak of Zika virus and its association with microcephaly, an up-to-date fetal head circumference (HC) nomogram is crucial to offer a reference standard in order to make an accurate diagnosis. This study was conducted to revise the local fetal HC nomogram.

MATERIALS AND METHODSIn this retrospective study, ultrasound data was used for construction of the fetal HC nomogram from a total of 6155 pregnancies in the ethnic Chinese population with low risk profile at KK Women's and Children's Hospital over a 10-year period. Regression model was fitted to calculate the mean and standard deviation of HC at each gestational age (GA). Comparison of HC between ethnic groups (no significant differences) and genders were made. The revised chart was compared with another commonly used reference chart (Hadlock). In an independent test population, different reference charts were used to estimate number of cases with microcephaly.

RESULTSA statistically significant difference of HC between the genders was observed across all gestational ages. Gender-specific reference charts and equation were computed. Our revised fetal HC chart showed a different distribution from the Hadlock chart. Compared with the gender-specific charts, the Hadlock HC chart would significantly under-report microcephaly cases in male fetuses, and tend to over-report in female fetuses.

CONCLUSIONThis study provides a new set of gender-specific fetal HC charts in the Singaporean population for antenatal ultrasound surveillance of microcephaly.