Loop leg is also known as O-shaped leg, the reasons are as follows: a. Physiological: O-shaped leg is a kind of pseudo O-shaped leg, mainly because the fetus in the mother’s tummy to maintain the bending position, so after birth the legs bending will be delayed for a period of time. Most of the O-shaped legs of babies within two years old are physiological, the newborn is born with inversion of the knee to two years old when it is close to normal, after two years old, gradually formed exotropia, 10 years old and then back to normal. Second, pathological: O-shaped leg is mainly rickets, hip dysplasia, if the infant lack of calcium, phosphorus, vitamin D, will cause calcium and phosphorus metabolism malfunction, common vitamin D deficiency rickets will lead to O-shaped leg. At this time, vitamin D and calcium should be supplemented, a reasonable and balanced diet, increased sunlight and exercise, gradually will improve. If your baby is born with unequal length of both lower limbs, hip joint popping, or limited hip abduction, or asymmetric hip pattern, you should be highly alert to hip dysplasia and need to seek medical attention in time.