Estimating fetal age by ultrasound measurements: an example of multivariate calibration.

A problem in the estimation of gestational age by ultrasound measurement of fetal bone lengths (the femur length, F, and the biparietal diameter, BPD) is described. By means of multivariate calibration techniques, F and BPD are combined to give an index of gestational age which is significantly more accurate than existing indices based on F or BPD alone. A detailed description of the statistical analysis is given, ranging from the initial steps of data analysis and model choice to the final step of determining the index and its accuracy. A number of interesting aspects of multivariate calibration theory are illustrated.