Breasts erupt at puberty and develop into mature type after 3~4 years; they have the function of lactation during pregnancy and start lactation after delivery. Breasts are the initial feature of the development of secondary sexual characteristics during puberty, around the age of 10, the level of sex hormones in the body rises, breast eruption, with the development of mammary ducts as the main, after 3~4 years of development into a mature type of mammary glands. During pregnancy, the placenta secretes a large amount of estrogen and progesterone, which promotes the development of mammary gland ducts and vesicles, and by the end of pregnancy, it already has the function of lactation, but there is no milk secretion yet. Human placental lactogen and pituitary prolactin are also involved in breast development during pregnancy. After delivery, the breasts, which are well developed during puberty and pregnancy, begin to perform lactation. The delivery of the placenta after delivery and the fetal nipple stimulate milk production. Breast development may be premature or underdeveloped. Women are advised to pay attention to breast development during puberty and pregnancy and to consult a doctor if there is any abnormality.