Abstract: The present invention pertains to a method for the prenatal detection of trisomy 21 in the second trimester by adjusting the risk of trisomy 21 based on fetal long bone biometry which comprises the steps of (a) measuring ultrasonically the biparietal diameter and the length of the femur, humerus, tibia, and fibula bones in fetuses of a patient population in the second trimester; (b) performing amniocentesis on the patient population in step (a) to determine which fetuses are normal and which fetuses have trisomy 21; (c) from the normal fetuses, deriving equations describing the predicted lengths of the lemur, humerus, tibia, and fibula based on the biparietal diameter measurements; (d) calculating a ratio of observed lengths to predicted lengths of the femur, humerus, tibia, and fibula for all fetuses; (e) comparing the ratios calculated in step (d) for normal fetuses against the ratios calculated for fetuses having trisomy 21 and determining a threshold, as a percentile of these ratios, for abnormally short