The breasts are hormonally regulated tissues that respond to a number of hormones circulating within the body. The developmental cycle of the breasts begins in puberty, and extends to changes during pregnancy. According to the Ohio State University Medical Center (OSUMC), the breasts do not fully mature until a woman has given birth and produced milk. During puberty, the breasts undergo periods of cell proliferation and division, which causes the breasts to enlarge and develop. During pregnancy, the cells of the breast proliferate to enlarge the breast, then transform into specialized cell types that make and secrete milk to allow for breastfeeding. After breastfeeding stops, the breast cells die and the breast reduces to its size to pre-pregnancy. Four hormones control the majority of these behaviors.
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Estrogen is the hormone that helps control breast cell proliferation and division. According to OSUMC, breast development during puberty begins after the ovaries start to secrete estrogen. This accumulates fat within the connective tissue of the breasts, causing the breasts to enlarge.
Estrogen is also released during the first half of the menstrual cycle, which enlarges the breast glands in preparation for pregnancy. Once estrogen levels decrease following ovulation, the breast returns to its normal state. If pregnancy occurs, the level of estrogen remains high to continue development of the breast in preparation for breastfeeding.
Progesterone is a steroid hormone that works together with estrogen to regulate breast development. According to the University of Virginia Health System, progesterone levels are low during the first half of the menstrual cycle, but contribute to breast development during the second half of the cycle, once estrogen levels are lowered.
Progesterone contributes to development of the breast by signalling for the formation of milk glands. Estrogen first induces enlargement of tissues within the breast, and progesterone ensures these tissues develop proper functioning within the breast to aid in breast development. If pregnancy occurs, progesterone levels remain high and control the development of the milk buds to prepare the mother for breastfeeding.
According to University of Utah College of Medicine, prolactin is a hormone secreted by the pituitary gland in the brain that stimulates lactation-secretion of milk from the mammary glands of the breast. Prolactin is secreted during the first few days of the menstrual cycle, with noticeable changes in the breast tissue as a result of prolactin noticeable the first few days of menstruation.
At the onset of pregnancy, prolactin levels decrease as estrogen and progestrone enlarge and develop the breast. Later in pregnancy, after the breast is developed and ready to secrete milk, prolactin levels are elevated. After birth, prolactin levels are stimulated by the suckling of the child, to allow for secretion of milk and breastfeeding.
Oxytocin plays a role in breast development during pregnancy. Like prolactin, oxytocin is secreted by the pituitary gland within the brain. According to Robert Wood Johnson University Hospital, oxytocin levels are high during the fifth and sixth months of pregnancy, and oxytocin helps stimulate milk production within the breast. The University of California reports that oxytocin also signals to the breast following childbirth to signal lactation in breastfeeding.