Charts of Thai fetal biometries: 1. Methodology.

A cross-sectional study was conducted in order to construct new reference charts for Thai fetal biometries that are commonly used in obstetric ultrasound practice. We discussed and illustrated a sound appropriate study design and statistical analysis which lead to more valid results. A total of 621 normal pregnant women between 12-41 weeks of gestation and their fetuses were recruited. Each fetus was measured once at a randomly assigned gestational age specifically for the purpose of this study only. Stepwise linear regression technique was used to model the mean and its standard deviation as functions of gestational age. Goodness of fit and normality of the data were checked before the final models were chosen. Reference centiles were derived, taking into account the increasing variation as pregnancy proceeds. We demonstrated the stated technique with humerus data from the same study. Reference charts for other fetal biometries have been derived and are presented in subsequent papers.