Breast development in early childhood is not uncommon in clinical practice. It is mainly caused by micropuberty, which is generally common in boys from one year to within one and a half years of age and in boys over half a year of age, and in the case of girls meet children from one year to within two years of age. Micropubescence is mainly caused by the hypothalamic-pituitary-gonadal axis being more active or partially activated, and children may develop breast development and breast enlargement without any obvious trigger, for example, in girls. In boys, the penis may become enlarged, or the penis and scrotum may become more pigmented. In general, if the hypothalamic-pituitary-gonadal axis is activated, it may also be due to increased sensitivity to sex hormones in the external genitalia. In the case of a girl, her follicles may secrete more estrogen transiently, or her aromatase activity may increase, or some external estrogen disruptors, such as the mother using estrogen-containing skin care products or eating estrogen-containing foods, and then breastfeeding her child, especially in girls, may cause breast development.