Ashley Alex’s research while affiliated with Virginia Commonwealth University and other places

The mature breast is located within the anterior thoracic wall, lying atop the pectoralis major muscle. Pubertal changes lead to incomplete development of the breast, a process which is only completed during pregnancy. The incomplete breast consists mostly of adipose tissue but also lactiferous units called lobes. These eventually drain into the lactiferous ducts and then into the lactiferous sinus and then to the nipple-areolar complex. During pregnancy, the breast undergoes both anatomic and physiologic changes to prepare for lactation. During the first trimester, the ductal system expands and branches out into the adipose tissue in response to the increase of estrogen. Elevated levels of estrogen also cause a decrease in adipose tissue and ductal proliferation and elongation. Estrogen also stimulates the pituitary gland which leads to elevated levels of prolactin. By the twentieth week of gestation, mammary glands are sufficiently developed to produce components of milk due to prolactin stimulation. Milk production is inhibited by high estrogen and progesterone levels and colostrum is produced during this time. In the third trimester and then rapidly after birth, these levels decrease, allowing for milk production and eventual let-down to allow for breastfeeding. Most pregnancies cause the areola to darken, the breast to increase in size, and the Montgomery glands to become more prominent. Post-lactational involution occurs at the cessation of milk production caused by a decline in prolactin.