The humerus and femur are often used by doctors to observe the development of the fetus during pregnancy examinations with ultrasound. Both the femur and the humerus are long tubular bones typical of the human body and can be divided into two ends of one piece. The femur is the largest long bone in the body and refers to the thigh bone. The humerus, on the other hand, is located in the upper arm and is also called the upper arm bone. As the name implies, femur length is the length of the bone of the thigh, and humerus length is the length of the bone of the upper arm. Both femur length and humerus length are fetal biological indicators that predict fetal body mass. However, fetal humerus length is superior to femur length in predicting fetal body mass.