Estrogen plays a key role in the development of breasts during puberty and pregnancy, and fluctuating estrogen levels in a woman’s lifetime also have an effect on their size and shape.
Estrogen comprises a group of three steroidal hormones: estrone, estriol and estradiol. These hormones are produced naturally in a woman’s body by her ovaries, adrenal glands, fat cells and, to a lesser extent, muscle cells. Throughout a woman’s lifetime, her natural estrogen levels experience periods of waxing and waning through puberty, pregnancy and menopause.
Estrogen contributes to the development of breasts during puberty and pregnancy, along with another type of hormone known as progesterone. Together, they send signals to the body that lead to maturation of breast tissues, ducts and glands. Estrogen also plays numerous other roles in the female body that affect the uterus, menstrual cycle, bones, muscles and some aspects of the liver and brain.
Men produce small amounts of estrogen, too, but it does not play a role in developing secondary sexual traits in them as it does in women.
Receptors located in the cells of breast tissues are stimulated by the presence of increased estrogen, and as a result the breast tissues expand and grow. Estrogen has a particular effect on the growth and maturation of duct tissue in the breasts, which are used during lactation and breast-feeding. Estrogen-fueled breast growth mainly occurs during puberty, throughout the years of physically maturing into an adult, and during pregnancy. During these times, estrogen production is increased in the body. When a woman reaches menopause, estrogen levels in her body decrease and the shape of her breasts may change from rounded to oval. Transgender men who want to become female typically receive hormone treatment, which includes estrogen, to develop their breasts, as the tissues in their chest also contain estrogen receptors.
While breast enlargement occurs as a result of increased estrogen it is not the only hormone that contributes to breast growth. However, many over-the-counter pills with synthetic and plant-derived estrogens are marketed as breast-enlargement products and may or may not work. Estrogen supplements and medications taken for such reasons as birth control may result in temporary breast growth, but the increase in bust size is mainly due to a temporary enlargement of the duct tissues and subsides once the individual stops taking the supplement or medication. In addition to estrogen, breast growth in women is dependent on progesterone levels, weight, diet, genetics and nutrition.
Women should never use estrogen supplements in an attempt to increase their breast size without talking to their physician. Long-term estrogen ingestion has been linked to increased risks of breast cancer, uterine fibroids, blood clots and uterine cancer.