Charts of Thai fetal biometry: Head circumference.

The purpose of this study was to develop reference centiles of head circumference (HC) for Thai fetuses, and to compare them with previously published data. Normal pregnant women who attend antenatal clinic at Siriraj Hospital provided the study population. A total of 621 cases between 12 - 41 weeks of gestation were recruited. Gestational age was determined by menstrual history which had been regular for at least 3 months without contraception prior to current pregnancy, and uterine size which was compatible with menstrual age at the first time of examination. Each fetus was measured once at a randomly assigned gestational age specifically for the purpose of this study. Due to unfavorable fetal position in some cases, HC data were only available in 609 measurements. Stepwise linear regression models were fitted separately to estimate the mean and standard deviation as functions of gestational age. A reference centile chart was constructed from both equations, assuming the data were normally distributed. A new reference centile chart for HC is presented and compared with previously published data. Our derived centile were lower than those from Western studies, which may partly be due to racial differences. This emphasizes the need to develop fetal biometric charts specific to each region.