Femur length versus biparietal diameter for estimating gestational age in the third trimester.

It is well established that ultrasound measurement of femur length and biparietal diameter are comparably accurate estimators of gestational age when obtained in the first half of pregnancy. Both estimators, however, become less accurate later in pregnancy. The present study compares the relative accuracy of these estimators when obtained in the third trimester. It is concluded from linear regression analysis that the correlation between gestational age estimated from femur length and the actual gestational age is stronger than that between gestational age estimated from biparietal diameter and actual gestational age. Furthermore, gestational age calculated from femur length is significantly more likely to be within 2 weeks (P less than or equal to .01) and 3 weeks (P less than or equal to .05) of actual gestational age than is gestational age calculated from biparietal diameter. Estimating gestational age from the mean of the gestational age based on biparietal diameter and that based on femur length is less accurate than estimating gestational age from femur length alone.