We retrospectively studied the radiographic documents of seventy girls with bony age varying from nine to eighteen years old and divided in nine equal groups and seventy boys with bony age from ten to nineteen years old. We analyzed the AP pelvic X ray of each child focusing on cartilages and ossification centers of both pelvic bone and upper femoral extremity. The purpose of this study was to assess the dates of apparition and ossification of these ossification centers and the dates of fusion of cartilages. The iliac ossification center (epiphysaris nucleus ilii) appears lately at bony age of thirteen years and half for girls and fourteen years and half for boys. The triradiate cartilage closes the earliest from its medial to its lateral portion before the apparition of the iliac ossification center. This closing date corresponds to the beginning of growth acceleration. Therefore, the period of growth acceleration can be identified by an accurate analyse of the triradiate cartilage closure on a single AP pelvic X ray.