The human breast begins to develop at six weeks’ gestation. At birth, the mammary gland contains only rudimentary ducts that have small club-like ends, which grow throughout childhood. During puberty the breast begins to enlarge through the formation of adipose tissue and the branching and elongation of the ductal system. However, it is during pregnancy that the most significant growth and development of the breast occurs.
Breast changes during pregnancy
In the first half of pregnancy secretory differentiation (the differentiation of alveolar epithelial cells into milk-secreting cells), ductal branching and lobular formation of the breast (mammogenesis) occur. Growth and development of the mammary tissues begins at around weeks three and four of gestation, with specific ductal branching and lobular formation. Proliferation of the distal portions of the ducts results in the formation of multiple alveoli (which contain lactocytes – cells that secrete milk).
Mammogenesis is influenced by a number of hormones, including progesterone, growth hormone and others, which enable the lobules to become fully differentiated. This differentiation may become obvious with the first appearance of colostrum during the secretory development phase in the second half of gestation. Here, the mammary gland is being populated with differentiated lactocytes that are able to synthesise milk components including protein, lactose, casein, α-lactalbumin and fatty acids in the form of colostrum (although only small volumes of colostrum may be available, approximately 30 ml/day). These changes are closely followed by heightened lobular growth and alveolar secretion late in pregnancy.
Throughout pregnancy there is significant growth of the secretory tissue, with approximately double the amount of secretory tissue compared with adipose tissue in the lactating breast. All of these changes usually lead to significant growth in breast and nipple size during pregnancy. However, breast growth varies largely between women, ranging from little or no growth to significant growth. For most women, increases in breast size occur by week 22 of gestation, although some women also experience significant breast growth into late pregnancy. Nonetheless, there is no relationship between breast growth and the ability to breastfeed or produce milk at one month post-partum.
Changes in the breast after weaning
After the baby has been weaned, the mammary gland undergoes restructuring to revert to a near pre-pregnancy state, ready to undergo the same cycle of development when the next pregnancy occurs. This process includes the removal of the milk-producing epithelial cells, whereby the alveolar epithelium undergoes apoptosis and is replaced with adipocytes.
Geddes, D. 2007, Gross Anatomy of the Lactating Breast. In: Hale, T. & Hartmann, P. (eds.). Textbook of Human Lactation. Amarillo, Texas: Hale Publishing.
Pang, W.W. and Hartmann, P.E. Initiation of human lactation: secretory differentiation and secretory activation. J Mammary Gland Biol Neoplasia 12, 211-221 (2007).
Cowie, A.T., Forsyth, I.A. & Hart, I.C. Hormonal control of lactation. Monogr Endocrinol. 15, I-275 (1980).
Cox, D.B. et al. Breast growth and the urinary excretion of lactose during human pregnancy and early lactation: endocrine relationships. Exp.Physiol 84, 421-434 (1999).
Kulski, J.K. and Hartmann, P.E. Changes in human milk composition during the initiation of lactation. Australian Journal of Experimental Biology and Medical Science 59, 101-114 (1981).
Chapman, D.J. et al. Impact of breastpumping on lactogenesis stage II after cesarean delivery: A randomized clinical trial. Pediatrics 107, E94 (2001).
Kent, J.C. et al. Breast volume and milk production during extended lactation in women. Exp Physiol 84, 435-447 (1999).