Corrected formula for uncertainty in estimations of gestational age from fetal head circumference measurements.

Over recent years, numerous publications have proposed methods for estimating gestational age (GA) using fetal measurements, including biparietal diameter, head circumference (HC) and crown–rump length, among others. The paper by Altman and Chitty1 presented statistical modelling of data from 663 fetuses to produce charts and tables for pregnancy dating based upon such measures. The resulting outputs were tables of estimates of mean GA based upon each measurement, each with a corresponding standard deviation that encompasses the uncertainty in the prediction. Here, we address an erroneous result in the appendix of this work, associated with the uncertainty in GA prediction based upon derived HC measurements. Altman and Chitty report the following formulae for calculating the mean loge GA (in weeks) and the associated standard deviation, as a function of (derived) HC: