Based on preterm and term infants studied by ultrasonography, and on anatomical sections of various gestational ages the physiological maturation of the hip joint is analysed. Current concepts of a linear growth pattern with an arrest immediately after delivery are confirmed. A more rapid growth and ossification of the acetabular edge than of the femoral head postpartum is suggested. To avoid overtreatment, knowledge of the normal range of development as seen in ultrasonography is mandatory.