Charts of Thai fetal biometries: 2. Biparietal diameter.

A cross-sectional study was conducted in order to construct a new reference chart for Thai fetal biparietal diameter (BPD). A total of 621 normal pregnant women between 12-41 weeks of gestation and their fetuses were recruited. Measurements were made once at a randomly assigned gestational age specifically for the purpose of this study only. Due to unfavorable fetal position in some cases, BPD data were available in 613 measurements. Linear regression models were fitted separately to estimate the mean and standard deviation as functions of gestational age. Reference centiles were constructed from both equations, assuming the data were normally distributed. A new reference centiles chart for BPD is presented and compared with previously published data. Our derived centiles were clearly lower than those from Western studies showing the importance of racial differences between populations. This elucidates the need to develop fetal biometries charts specifically for each region.