Ultrasound measurement of fetal limb bones.

A study was made of 41 patients with known menstrual dates in whom the duration of gestation, as determined by physical examination and ultrasound scan, corresponded with those dates. Examination of the fetal limb lengths was done every 1 to 3 weeks, starting at 8 weeks' gestation. A Kretz Combison 100 ultrasound sector scanner with a 2.5 MHz transducer (velocity = 1,540 meters per second) was used. A freeze frame was used when the full bone length was visualized, and then electronic calipers were employed to measure its full length. Serial measurements of the humerus and femur and the radius-ulna and tibia-fibula complexes were made. The values were expressed as means +/- 2 standard deviations for each week of gestation. The growth of the fetal limb bones was linear from 12 through 22 weeks' gestation, but the various bones appeared to grow at different rates. The femur was the first to be well defined and the easiest to measure with reproducibility. All of the limb bone lengths correlate with gestational age and may serve as indicators of skeletal dysplasia. A patient who was delivered of an infant affected with diastrophic dwarfism syndrome was diagnosed at 16 weeks' gestation to have a fetus affected with this problem. Two other patients whose pregnancies were at risk for skeletal dysplasias were correctly diagnosed to be normal.