Because the breasts are principally composed of adipose tissue, which surrounds the milk glands, their sizes and volumes fluctuate according to the hormonal changes particular to thelarche (sprouting of breasts), menstruation (egg production), pregnancy (reproduction), lactation (feeding of offspring), and menopause (end of menstruation). For example, during the menstrual cycle, the breasts are enlarged by premenstrual water retention; during pregnancy the Approximately two years after the onset of puberty (a girl's first menstrual cycle), the hormone estrogen stimulates the development and growth of the glandular, fat, and suspensory tissues that compose the breast. This continues for approximately four years[clarification needed] until establishing the final shape of the breast (size, volume, density) when she is a woman of approximately 21 years of age.[10] About 90% of women's breasts are asymmetrical to some degree,[10] either in size, volume, or relative position upon the chest. Asymmetry can be manifested in the size of the breast, the position of the nipple-areola complex (NAC), the angle of the breast, and the position of the inframammary fold, where the breast meets the chest.
For about 5%[10] to 10%[17] women, their breasts are severely different, with the left breast being larger in 62% of cases.[17] This is due to the breast proximity to the heart and a greater number of arteries and veins, along with a protective layer of fat surrounding the heart located beneath it.[18] Up to 25% of women experience notable breast asymmetry of at least one cup size difference.[10][19][20][21]
If a woman is uncomfortable with her breasts' asymmetry, she can minimize the difference with a corrective bra[18] or use gel bra inserts.[18] Alternatively, she can seek a surgical solution. Options include a minimally invasive procedure known as platelet injection fat transfer, which transfers fat cells from a woman's thighs to her smaller breast.[10] More invasive