Children who have new teeth before their teeth fall out are considered to have retained milk teeth. There are many reasons for retained milk teeth, generally it may be due to abnormal root resorption of milk teeth, abnormal eruption of inherited teeth and systemic factors. 1. Abnormal root resorption of milk teeth: due to genetic or external factors, the direction of eruption of inherited permanent teeth is abnormal, or the eruption of inherited teeth is weak, resulting in incomplete root resorption or slow resorption of milk teeth, which prevents milk teeth from falling out. 2. Abnormal eruption of inherited teeth: The inherited teeth cannot be lost due to ambiguous obstruction or ectopic eruption, which results in the formation of stagnant milk teeth. 3. Systemic factors: Certain systemic diseases, such as dwarfism, rickets, hypothyroidism, etc., can lead to retained milk teeth. There are other causes of retained milk teeth, so it is recommended to consult a doctor to find out the cause and follow the doctor’s instructions to start treatment.