In the first few months after the baby is born, the muscles and bones of the hand are still developing, and the tension of the flexor muscles leads to thumb buckle. With the baby's growth, muscle tension and bones gradually mature, flexors gradually relax, and thumb adduction gradually disappears. Generally speaking, when most babies are 3 to 4 months old, the phenomenon of thumb adduction will be significantly reduced or disappeared. There are also a few babies who may take longer to disappear completely, but usually it will not exceed 6 months.