Growth of fetal limb bones has been examined in a prospective cross-sectional study in 530 patients between 13 and 42 weeks of gestation by ultrasound. The length of the femur, tibia, humerus, and radius were measured in all cases, and fibula and ulna in 393 cases. For each week of gestation mean lengths (+/- 2 standard deviations) were calculated. All limb bones showed linear growth from 13 weeks to 25 weeks of gestation, after which the growth curve appeared nonlinear. A strong linear relationship (r greater than 0.98) between the BPD and bone length was found for each bone. In five aborted fetuses, prenatal sonographic measurements of the ossified diaphyses were compared with the postnatal radiologic measurement (30 limb bones). A good agreement was found (difference between ultrasound and X-ray measurements 0.7 +/- 0.5 [SD] mm or 3.0 +/- 2.4 [SD]%), indicating that ultrasound images the ossified diaphysis.