At present a large number of different charts are used for prediction of gestational age from sonar biparietal diameter (BPD). In this report the reasons for these observed differences are presented. Additionally, the usefulness of all these charts is questioned because a) the mean differences in 7059 BPDs derived by the B-scan from four large fetal population studies are not significantly different from zero and b) BPDs obtained by B scan are statistically comparable to gray-scale or real-time BPDs if medium gain is used. Thus, it is our suggestion that a chart showing the composite mean BPD values of all four studies be used universally for prediction of fetal age. Finally, the guidelines of using sonar BPD as an index of gestational age are presented and the role of the obstetrician in interpreting BPD data is emphasized.