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Breast Changes In Pregnancy

What Role Do Hormones Play?


Updated June 02, 2014

For your body to ready itself to breastfeed, many different changes occur in your breasts during pregnancy. Major hormonal influences take place during this time. Here is everything you need to know about the process.

Stages of Mammary Function

  • Mammogenesis: This includes mammary development, which begins in the fetal stage and speeds up during puberty with further growth and enlargement of the breasts during the menstrual cycle and pregnancy. However, the breast is not entirely developed until after it has produced milk.
  • Lactogenesis (Stages I, II, III): This is the beginning of milk secretion. The very significant Stage II starts at about 28 weeks gestation. You may start to realize that you are leaking colostrum, which is held in the ducts during the second trimester of pregnancy and is secreted during the initial postpartum period. This phase endures only if the breast is sufficiently stimulated -- then, in a few days time, the colostrum slowly changes to transitional milk. Keep in mind that hormonal shifts are intense during this period. Stage III starts at about 10 days postpartum. This is the beginning of a mature milk supply, which is made up of foremilk and hindmilk.
  • Lactation: The period following birth in which milk is secreted. This lasts until weaning occurs.
  • Involution: The mammary gland returns to its nonproductive state of milk secretion. Essentially, milk is no longer produced.

Breast Changes During Pregnancy

The breasts go through further changes throughout pregnancy and postpartum in order to sustain lactation. Breast changes begin following conception and the weight of the breast increases approximately 12 ounces during pregnancy. In addition, bloodflow to the breast doubles.

External Changes In The Breast

The pregnant breast will increase in size and the areola will darken. Also, the skin will seem thinner and the veins are much more noticeable. The Montgomery glands (tiny little bumps on the areola which secrete natural oils to lubricate the area and help to prevent bacteria from breeding) become larger and more noticeable as well, and the nipples become more pronounced.

What Changes Occur During The First Trimester?

The ductal system expands and branches under the influence of estrogen. The glandular tissues of the alveoli (tiny sacs in which milk is stored and secreted) also expand, but this is in response to hPL, hCG, and prolactin. Progesterone helps to increase the size of the lobes and lobules (the branching network of the internal structure of the breast; each breast has approximately 20, ending in the nipple.) Growth Hormone and ACTH (adrenocorticotropic hormone, which stimulates the adrenal glands) work together with prolactin and progesterone to foster mammogenesis. Also during this time, lymphocytes, plasma cells, and eosinophils are gradually introduced into the small, narrow spaces between the tissue of the breast, or interstitial tissue.

What Changes Occur During the Second Trimester?

There is more development and enlargement of the duct system and additional growth of the lobules. From about the third month, a secretory substance that is similar to colostrum materializes in the acini, or milk-producing cells. From that point on, prolactin (from the anterior pituitary gland) triggers the glandular production of colostrum and placental lactogen starts to prompt the secretion of colostrum.

What Changes Occur During the Third Trimester?

During this period, there is even further lobular development and growth of additional alveoli and ducts (which also begin to dilate). The epithelial cells of the alveoli separate into secretory cells, which are able to produce and release milk, and where fat globules collect. Further dilation of the breast is triggered by the rise in secretory cells and expansion of the alveoli with colostrum. Soon before and during birth there is an additional occurrance of mitotic action, which boosts the total DNA of the gland.

What Changes Occur Postpartum?

After birth, milk is formed and it is released into the milk-producing cells and ductal system. The two most critical hormones involved in milk production are prolactin and oxytocin, and as long as milk is withdrawn frequently from the breast, the alveolar cells will continue secreting milk for an indeterminate period of time.


Riordan J, Auerbach KG. Breastfeeding and Human Lactation. Jones and Bartlett. 2nd Edition.

Lawrence RA. Breastfeeding: A Guide For The Medical Profession. Mosby. Fifth Edition.

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