It takes about three months to six months for a girl to develop her mammary glands and have her first menstruation, mainly because the length of this time varies with individual differences. The vast majority of girls get their periods between the ages of 13 and 15, and due to individual differences, the time of menstruation varies, as long as it is between the ages of 9 and 18 it is normal. If breast development occurs at the age of less than 9, it can be clinically diagnosed as precocious puberty, and it is recommended to go to the hospital for further examination. In addition to dietary adjustments, it is also recommended that medication may be required if necessary. Therefore, parents should carefully observe when a girl’s breast development occurs, and then clinically understand whether it is a normal period of breast development. In addition, once a girl has her period, it is important for parents to provide good care guidance.