The median nerve passes through the humeral and ulnar heads of the pronator teres muscle (PT), although variations such as absence of the ulnar head may exist. We observed histological sections of the upper extremity from 24 embryos and fetuses. In the early stage, the PT extended between the radius and the medial epicondyle of the humerus, but no candidate for the ulnar head was found. In mid-term fetuses, the ulnar margin of the PT was attached to the elbow joint capsule. Moreover, in late-stage fetuses, a small deep part of the PT arose from the thick joint capsule of the humero-ulnar joint near the coronoid process of the ulna. This joint capsule also provided the most proximal origin of the flexor digitorum profundus muscle. Therefore, we considered fetal PT origin from the capsule as a likely candidate for the ulnar head. Consequently, the PT seemed to develop from a single anlage through which the median nerve passed, but later – possibly after birth – a small PT origin from the joint capsule appeared to obtain an aponeurosis connecting the muscle fiber to the ulna. This secondary change in PT morphology might explain the muscle variation seen in adults.