To determine the baby’s O-legs, physical examination, X-ray examination and medical history can be used. 1. Physical examination: put the patient in a supine position, straighten both lower limbs together, with the inner ankles close together, if there is a gap between the knee joints and the knees can not be brought together, it is more likely that the baby has O-shaped legs. 2. X-ray examination: X-ray examination of both lower limbs, whether the line between the center of the femoral head and the center of the knee joint is in the center of the ankle joint, for diagnosis. The angle between the long axis of the tibia and the baseline of the femur and knee can be used to check the degree of knee entropion. 3. Medical history: If the patient has a history of rickets or vitamin D deficiency, he or she is more likely to have O-legs. Patients should seek medical attention if they experience discomfort.