Two growth curves of the fetal biparietal diameter are presented. One is prospective, based on the last menstrual period, while the other is retrospective, calculated from the date of birth. The prospective curve shows a slight decline in average values following the wk 42 of gestation. This is thought not to reflect a true shortening of the biparietal diameter in the single fetus, since serial measurements in single fetuses show growth up to the time of delivery. Individual growth curves, grouped according to the weight-for-dates of the newborn child, show a trend towards longer biparietal diameters in the larger fetuses. Serial measurements, preferably starting before wk 30, appear to be a good help in predicting birth weight, but growth rates without regard to the actual measurement values were of little prognostic value in the present material.