Women experience basic maturation of the mammary glands for 4 to 6 years around the age of 9. The mammary glands that make up the breast and the fatty tissue that protects them form a small button-like bulge at the breast and its surrounding areola (the bud stage), and thereafter, from the flattened breasts of early childhood to slightly prominent nipples. Later, the breast gradually bulges and increases in size, the nipple becomes larger, the areola gradually expands, the color of the breast areola gradually deepens, the outer shape of the breast resembles a hemisphere, plump and upright, located on the surface of the pectoralis major muscle in front of the second to sixth rib. 16 to 17 years old, the breast is plump and well-defined, the areola is slightly sunken, and the nipple is large and prominent. 18 to about 20 years old, the size and shape of the breast is already adult female (breast stage). Microscopically, breast lobules, breast follicles, breast ducts, subcutaneous tissues and breast suspensory ligaments develop and grow under the action of estrogen secreted by the ovaries, preparing for future milk secretion. During this period, the normal breast is unevenly developed in different parts of the breast due to the uneven sensitivity of the breast tissue to hormones, which may result in a feeling of swelling and pain. In addition to an increase in the diameter of the areola, 60-70% of boys can have small hard nodules under the breast that are tough and accompanied by a light base of tenderness, which usually disappear within 1-2 years.