In fact, "sexual maturity" has gone through four stages: fetal period, perinatal period, childhood and adolescence. When the fetus is in the mother's womb, because the placenta is connected, it doesn't need to supply estrogen to promote its development, and its mother "helps". However, once the fetus leaves the umbilical cord and becomes a newborn, the gonadal hormone needs to be "self-sufficient". The axis of hypothalamus-pituitary-gonad will be established immediately in the baby, which is responsible for the secretion of estrogen and androgen. This stage is called "perinatal infancy".

However, before the age of two, this system is unstable, and it is still in the "debugging stage". Some girls will intermittently secrete estrogen, and the level can even reach the low limit of estrogen for normal adult women. Girls who are sensitive to estrogen will have breast development, so it is called "puberty".

Generally, girls may have large breasts within two years of age, but most of them will subside by themselves. If they do not subside after two years of age, it is recommended to see an endocrinologist.