How Your Breasts Change To Prepare For Breastfeeding

Mammary Development, Function, And Changes In Pregnancy

Breast Changes In Pregnancy and Postpartum To Prepare For Breastfeeding
How do your breasts change to prepare for breastfeeding?. Henrik Sorensen/The Image Bank/Getty Images

How Your Breasts Change To Prepare For Breastfeeding

When you're pregnant, and your body is preparing to breastfeed, your breasts go through many different changes. The shifts in major hormones that takes place during pregnancy influence the growth and development of your breasts. Here is everything you need to know about the process.

The Stages of Breast (Mammary) Development and Function

  • Mammogenesis: Mammogenesis includes breast development, which begins before you're born and speeds up during puberty. Further growth and enlargement of the breasts occur during the menstrual cycle and pregnancy. However, the breasts are not entirely developed until after they have produced breast milk.
  • Lactogenesis (Stages I, II, III): Lactogenesis is the beginning of breast milk secretion. The very significant Stage II starts at about 28 weeks of pregnancy (gestation). About this time, you may begin to realize that you are leaking colostrum. Colostrum is held in the milk ducts during the second trimester of pregnancy, and it's secreted during the initial postpartum period. This phase of breast milk production will only continue if the breasts are sufficiently stimulated -- then, in a few days time, the colostrum slowly changes to transitional breast milk. Keep in mind that hormonal shifts are intense during this period. Stage III of lactogenesis starts at about 10 days postpartum. It is the beginning of your mature breast milk supply, which is made up of foremilk and hindmilk.
  • Lactation: The period following the birth of your child when breast milk is secreted is called lactation. Lactation lasts until weaning occurs.
  • Involution: Involution is the return of the breasts (mammary glands) to their nonproductive state of milk secretion. Essentially, this is when the production of breast milk stops.

How The Breasts Change During Pregnancy

Your breasts go through changes throughout your pregnancy and the postpartum period in order to sustain lactation.

Breast changes begin after conception, and the weight of the breast increases approximately 12 ounces during pregnancy. Additionally, blood flow to the breast doubles.

As your pregnancy progresses, your breasts will increase in size, and the areola will darken. The skin surrounding your breasts will seem thinner, and the veins will be much more noticeable. The Montgomery glands (tiny little bumps on the areola that secrete natural oils to lubricate the area and help to prevent bacteria from breeding) will become larger and more noticeable, and the nipples will become more pronounced.

How Do The Breasts Change During The First Trimester?

During the first trimester of pregnancy, the ductal system expands and branches out under the influence of estrogen. The glandular tissues of the alveoli (tiny sacs in which breast 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.

How Do The Breasts Change During the Second Trimester?

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.

How Do The Breasts Change During the Third Trimester?

During the third trimester, 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 can produce and release breast 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 occurrence of a mitotic action, which boosts the total DNA of the gland.

How Do The Breasts Change After Your Baby Is Born?

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


Lawrence, Ruth A., MD, Lawrence, Robert M., MD. Breastfeeding A Guide For The Medical Profession Seventh Edition.  Mosby. 2011.

Riordan, J., and Wambach, K. Breastfeeding and Human Lactation Fourth Edition. Jones and Bartlett Learning. 2014.

Edited by Donna Murray

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