Estimation of gestational age from measurements of fetal long bones.

The general practice of using fetal long bone growth charts to derive gestational age does not yield mathematically valid results. The authors tried to calculate gestational age from the lengths of long bones (femur, humerus, tibia, ulna) from 12 to 40 weeks of gestation. The combined use of the four bones allows a good estimation of gestational age that may be useful should the biparietal diameter measurement be unreliable, unobtainable, or abnormal.