Embryology (How does it happen?)

Limb development occurs during the first 2-7 weeks of fetal life. The limb buds start to develop by 32 days. The most proximal (toward center) parts of the limbs develop first, followed by the hands and feet, which are fully formed by the seventh week.[Westin, 1969] The acetabulum (hip socket) and the femoral head and neck develop from a common block of cartilage, with subsequent cleft (opening) to create a hip joint cavity.[Abdel-Mota'al]

The cleft gradually separates until a definite joint cavity is formed. This point is critical for proper diagnosis. Dr. King pointed out that the development of the acetabulum and femoral head are mutually dependent (can't have one without the other). The presence of an adequate acetabulum on early (1st year) X-rays always means that the femoral head is also present - even though the femoral head's development may be delayed and the femoral head itself may not be visible on the X-ray. Similarly, if no acetabulum can be seen, no femoral head will develop.[Rajbich, 1991]

Whatever affects the developing limb bud during the first 2-7 weeks can also affect muscle development in the thigh, knee joint, and sometimes in the lower leg.[Rajbich, 1991]