Milk molar teeth are normally replaced, and when they fall out automatically, inherited permanent teeth will take the place of the original milk molar teeth. If a retained molar occurs, it can cause a delay in the replacement of the molar. Molar teeth include the first and second molar, the first molar is usually replaced at the age of 11 to 12 years old, and the second molar is replaced at the age of 11 to 13 years old. The exact timing of tooth replacement will be affected by the state of general development, nutrition, and genetic factors. If the milk molar is not lost in time, it is called retained milk teeth, and there are four situations, as follows 1. Inherited permanent teeth have already erupted and the milk molar has not been lost, forming a “double row of teeth”. 2. Inherited permanent teeth have insufficient eruption power, or developmental abnormalities, resulting in the failure to erupt on time, and the molar teeth do not fall off. 3. The inherited permanent teeth are congenitally missing, and the molar teeth do not fall off, so as to replace the missing permanent teeth to exercise the function. 4. Certain systemic factors or genetic factors lead to retained milk teeth, and the specific mechanism is not yet clear. If the milk molar is not lost after a certain period of time, or accompanied by other symptoms, you should go to the hospital in time to get a clear explanation of the cause and then give targeted treatment.