One leg is thick and the other is thin, which may be normal due to congenital development, or may be caused by diseases such as poliomyelitis and hip dysplasia. 1. Congenital development: when a baby has a thick leg and a thin leg, it is considered to be normal when the degree is slight. Because human limbs are not exactly the same, the phenomenon that two legs are not the same thickness is a normal physiological phenomenon. 2. Poliomyelitis: Poliomyelitis is caused by poliovirus infection, and children will have fever, irritability, vomiting, paralysis, limb muscle atrophy and other symptoms. Polio can cause muscle atrophy in one of the infant’s legs, resulting in the phenomenon of one leg being thick and the other thin. 3. Hip dysplasia: The cause of hip dysplasia is not yet clear, and it is considered to be caused by a variety of factors such as genetic and mechanical. The main symptoms of the affected children are unequal length of the two lower limbs, activity disorders, joint popping, asymmetry of the hip line, etc., which can also lead to the baby’s one leg thick and one leg thin. There may also be other reasons for infants to have one leg thick and one leg thin, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or treatment under the guidance of the doctor.