Most are unilateral and usually are on the left. In utero dislocation of hip is unusual.
Most dislocations occur after birth and are related to unstable or dislocatable hips.
Ultrasound of the hips is very useful in making the diagnosis in the newborn. Femoral head ossification centers appear at 3-6 months so radiographs are not useful in the newborn. By 2-4 months old you begin to see lateral displacement of the femur and an increased acetabular angle. In older children on pelvic films superolateral displacement of the proximal femur, increased acetabular angle, delay in appearance of capital femoral ossification center, loss of the lateral margin of acetabulum, abnormal sclerosis of acetabular rim, and formation of a false acetabulum are seen. See References Chapter.