Anatomic study of the pelvis of 30 fetuses and newborn children, yielded a precise relationship between the sacrotuberous ligaments and size of the gluteal region. At birth, the distance between the posterior superior iliac spine and the ischium is 40 mm. Closure of the anterior defect in bladder exstrophy sometimes requires an orthopedic surgical procedure. Bilateral posterior iliac osteotomy and section of these ligaments can be easily done by a single medial posterior approach herein described.