We measured the accuracy of second- and third-trimester sonographic predictors of gestational age against highly reliable gold standard (crown-rump length) in a group of fetuses. Using a prospectively collected computerized data base, we selected 460 fetal sonograms obtained at 14-42 weeks of gestation in which age could be reliably established on the basis of crown-rump length in the first trimester. We used data obtained from these sonograms to compare several predictors of fetal age. The accuracy of all predictors worsened progressively as pregnancy proceeded. In the second trimester, corrected biparietal diameter and head circumference were more accurate predictors of gestational age than were biparietal diameter, femoral length, and abdominal circumference (p less than .05, F test). In the third trimester, the corrected biparietal diameter, head circumference, and femoral length were the best predictors, significantly better than biparietal diameter and abdominal circumference (p less than .05, F test). Prediction of gestational age that relies on a single sonographic measurement should be based on the head circumference or corrected biparietal diameter in the second trimester and on one of these two predictors or the femoral length in the third trimester.