Six out of ten newborns will experience some degree of physiological activity in the mammary glands, such as swelling and hardness under the nipples and milk-like discharge from the nipples, which usually appear 3-4 days after birth and disappear after 1-3 weeks, due to maternal hormones entering the baby’s This is due to maternal hormones entering the baby’s body. The puberty period of women is the stage when sexual changes begin to mature, which lasts about 2-5 years. It is generally believed that the mammary glands begin to develop 3-5 years before the onset of menstruation. Girls in China begin to develop their mammary glands at the age of 9-15 years old, with an average age of 11.4 years old, but girls who often consume beverages and foods containing hormones (such as feed-fed chickens) often develop their mammary glands earlier. Generally, by the time the mammary glands are mature, 1/3 of the population is still without menstruation. The onset of menstruation is a sign that the sexual organs and mammary glands are fully mature. When a woman’s mammary glands begin to develop, the entire breast, areola, and nipple increase in size one after another, and the color of the nipple and areola deepen. The development of the mammary gland is uniformly conical, and the nipple is generally proportional to the development of the areola, but the development of the areola is more closely related to the development of the mammary gland. The enlargement of the entire mammary gland during this period is mainly due to the increase in fibrous tissue and subcutaneous fat. Some girls may have breast pain, but as they get older, their pain can be relieved. All of these changes occur under the influence of estrogen. If estrogen stimulation is too strong, it can lead to overall enlargement of the breast or the formation of localized “fibroadenomas”. In some boys, the mammary glands are more prominent than in females, and hard nodes the size of buttons can be palpated under the nipples, with mild pain, usually disappearing after 1 or 1.5 years. The relationship between menstruation and the development of the mammary glands is very close, the pre-menstrual period is larger, swollen, tough, small nodules when touched, sometimes with mild pain and pressure, and even a small amount of nipple overflow, after menstruation to reduce or disappear. After menstruation, the premenstrual phase of breast enlargement subsides, but fatty and connective tissues are abundant. In some cases, the hyperplasia does not degenerate and recover, forming ”mastocytosis.” In women who are not pregnant after the age of 30, the development of their lobules often becomes irregular due to the endocrine disharmony that often occurs during the cycle, but most of the glandular lobules proliferate and a few remain in a degenerative and recovered state. The mammary glands change significantly during pregnancy. After the 5th to 6th week of pregnancy, the mammary glands begin to increase in size, with the most pronounced increase in mid-pregnancy, when subcutaneous varicose veins are visible, sometimes with white lines on the skin, while the nipples increase in size and the areolas enlarge. The epidermis is thickened and there are 12-15 elevations in the areola, which are the location of the areolar glands, which resemble sebaceous glands and begin to secrete sebum in preparation for infant nursing. The changes in the various parts of the mammary glands are not uniform, with some developing more rapidly, some more slowly, and some not even seeing development, but can be fully developed during pregnancy. This imbalance in development may lead to cystic lesions in the breast in the future, and those who do not have most of their mammary glands fully developed will have insufficient milk production during lactation. Colostrum can be seen in the middle of pregnancy, but formal lactation begins 1 – 4 days after delivery. During the postpartum period to formal lactation, the mammary glands are obviously swollen and hard, with varying degrees of distension and pain. Once breastfeeding begins, the swelling and pain disappear. The amount of milk secretion is related to the degree of development of the mammary gland lobules during pregnancy, and even in the same person, the amount of secretion from the left and right mammary glands is not the same. The mammary glands generally return to their original state after a few months of weaning, but residual milk secretion is common and may occasionally persist for several years, with those with residual milk secretion being prone to secondary infection. Pregnancy and lactation can accelerate the development of benign or malignant breast tumors, but can cause cystic hyperplasia to subside. The breast gland is still fatty due to fat deposition, but the gland is shrinking and the fibrous tissue is increasing significantly. The development of the mammary gland is extensive in women with multiple births, but limited and abnormal in women with few births or no births. In women aged 30-40 years, abnormal breast development, such as cystic hyperplasia, is seen in one third of cases.