Under normal circumstances, children replace all of their baby teeth, but some children may also have milk teeth that are left behind. There are two sets of teeth, the baby teeth and the permanent teeth. The baby teeth erupt between six months and two and a half years of age and are replaced between six and twelve years of age. However, it is common for some of the baby teeth not to be replaced in clinical practice, which is called retained milk teeth. The reasons for this include congenital absence of some permanent teeth or late eruption of permanent teeth and inflammation of the root tip of the milk teeth prior to eruption that inhibits the eruption of permanent teeth. In addition, crowding of the teeth, which does not allow enough space for the eruption of the permanent teeth, can also lead to retention of the baby teeth. Parents should pay attention during the tooth replacement stage, if the child’s baby teeth have not fallen out and the permanent teeth have already erupted, it is recommended to take the child to remove the retained baby teeth quickly, otherwise the permanent teeth may be distorted and not return to their normal position, affecting the effect of tooth alignment.