A girl’s breasts will probably develop at the age of about ten years old. At the age of ten to eleven years old, the breasts gradually swell up, such as small mounds, while the areola area expands, indicating that the mammary glands are beginning to develop. At the beginning of development, for example, hard knots will appear in the place of both breasts of girls, and the child may feel pain, all of them are normal phenomena. It is also normal for some girls to develop on one side first, rather than both sides at the same time. Girls start to develop breasts, which slowly get bigger and bigger, followed by pubic and armpit hair. After about six months of breast development, girls may gain height more quickly, which is normal. When breast development is about two and a half years old, the child may have a period, which is a normal part of a girl’s development. If breast development occurs before the age of eight, it is diagnosed as precocious puberty, defined as a developmental abnormality. Clinically, it is important to find out the cause and effect of the developmental abnormality on the child. Blood, bone age films and color Doppler ultrasound can be taken to check and intervene if necessary.