Teeth begin to erupt six months after birth and erupt one after another by about two and a half years of age, with a total of 20 teeth, 10 on top and 10 on bottom, symmetrical on the left and right. From the age of 6 to 7 years old to 12 to 13 years old, the milk teeth gradually fall out and are replaced by permanent teeth. I. Development of teeth at embryonic age: When the mother is six weeks pregnant, the baby’s milk teeth already start to develop. First, the epithelial cells in the oral cavity differentiate into tooth germ, which undergoes a series of complex and special changes, and then forms various special shapes of teeth according to their locations. Once the embryo has reached a certain stage of formation, the tooth begins to calcify and harden due to the deposition of minerals such as calcium and phosphorus. After the crown calcification is completed, the root of the tooth is formed, and when the root is nearly three-quarters complete, that is, when the normal tooth emerges from the gums. (So the baby late teething is not a baby calcium deficiency, but a lack of calcium caused by the mother’s pregnancy.) Second, the development of teeth at birth: when the teeth begin to calcify, you can see the teeth through X-ray photography. People generally think that newborns do not have teeth, in fact, when babies are born, if the X-ray photography, you can see not only all the 20 milk teeth, and even part of the permanent teeth (such as the first molars), but they have not yet erupted into the mouth. The eruption time and order of milk teeth: The eruption of teeth has an approximate time, which is not fixed, and usually a difference of six months is considered normal. Before the age of one, the order of eruption is as follows: the two lower central incisors – the two upper central incisors – the upper lateral incisors adjacent to the central incisors – the lower lateral incisors After the age of one, the order of eruption is as follows: the two lower first molars – the two upper first molars -Cuspids between the lower lateral incisor and the first molar -Cuspids above -Two second molars below -Two second molars above