Women's Health Series // The Physiology of Breastfeeding

These Women's Health Series posts have been the most interesting and exciting ones that I think I've been able to put together to share with all of you often the two and a half years of running this blog. I am just amazed at how wonderfully we have been created, how our bodies function, and how we have been equipped to carry out what may seem like ‘basic’ functions (yet are so complex in the way that they actually work) with grace. After spending a bit of time reading through Robin Kaplan's amazing book 'Latch: A Handbook for Breastfeeding with Confidence at Every Stage’ *, I thought I might as well do a basic post on the physiology of breastfeeding to just cover the basics of how it works as part of the Taste & See Women's Health Series. Once again, the idea behind these more foundational posts is to set the scene for others that I aim to write in future, in this case particularly as I enter into a phase of life where I will be navigating my own breastfeeding journey for the first time. I hope that you find the information that I have shared in this post as AMAZING as I did as I was learning more about how the anatomy of the breasts and physiology of lactation work to nourish a growing baby. Enjoy!

Breastfeeding Anatomy

Illustration by  Duvet Days

Illustration by Duvet Days

Lobules or Alveoli - These structures, which look a bit like grapes on a vine, are responsible for producing and storing milk.

Ducts - These are the tubes that are responsible for carrying milk from the alveoli to the nipple.

Nipples - These each have many small openings where milk comes out.

Areola - These are the pigmented areas on the breast surrounding the nipples. During feeding, it is important that as much of the areola as possible is in the baby’s mouth for effective transfer of milk out of a woman's breast.

The Montgomery glands - Not pictured here, but these are a sebaceous and mammary milk glands that are believed to produce secretions that:

  • Lubricate the nipple and areola to protect them from mechanical stress due to sucking

  • Protect the nipple against infection

  • Produce volatile compounds that act as an olfactory stimulus that help a newborn find the breast and stimulate their appetite

How Breastfeeding Works

  1. Milk Production

There are two stages of growth that occur as breasts start to prepare for breastfeeding during pregnancy. During the first half of pregnancy, the process of mammogenesis (lobular and alveolar growth) results in an increased number and size of the alveoli. There is extension and branching of the ductal system that takes place at this point, and the skin on breasts tends to become thinner and the veins become more visible due to the influence of prolactin and hCG. It is this intense growth that can cause the breast tenderness that women experience during early pregnancy. The process is mainly driven by increased production of hormones oestrogen and progesterone, which are responsible for promoting the development of breast tissue, multiplying milk ducts, and thus preparing a women for lactation after pregnancy. Other hormones that play a role in mammary tissue growth include growth hormone (GH), insulin-like growth factor (IGF), and fibroblast growth factor. During the second half of pregnancy, prolactin levels increase drastically. This promotes the formation of alveoli on the milk ducts and more branching of the ductal system. By mid-pregnancy a woman's breasts may begin to feel heavier as colostrum is produced, and the nipples and areolae continue to darken and enlarge.

2. Milk Release

Once a baby is born, milk volume continues to increase and will eventually need to be released. Prolactin is responsible for stimulating and supporting lactation. Milk is released through milk ejections (also known as 'letdowns') that happen as a baby suckles on the breast. The process of milk ejection is very important for successful lactation, because until this happens only small volumes of milk (1-10ml) can be expressed. The milk ejection process is initiated when a baby's tongue stimulates the nipple, which sends nerve impulses to the hypothalamus, which causes the posterior pituitary gland to release oxytocin into the bloodstream. Oxytocin causes cells surrounding the alveoli to contract and squeeze breastmilk out into the ducts, which travels down the milk ducts and out of the nipple openings. The let-down reflex is driven by a positive feedback loop (as pictured below) whereby suckling continues to stimulate oxytocin release, which increases milk production and release, and will continue until the baby stops suckling. This reflex can be triggered by a mother’s thoughts about her baby or by her baby’s cry, and can be inhibited by things like feelings of anxiety and insecurity, tiredness, and a mother being in pain.

The positive feedback loop that responsible for the let-down reflex

The positive feedback loop that responsible for the let-down reflex


3. Milk Transfer

Milk transfer refers to the movement of milk from a woman's breast into her baby's mouth so that it can be swallowed. Researchers have recently shown that milk flow into an infant's mouth occurs when the tongue is lowered and vacuum in the mouth is increased. Suckling needs to continue between letdowns to ensure that milk keeps flowing. A baby needs to have a deep and effective latch to create sufficient negative pressure inside of its mouth whilst suckling to remove milk successfully. Milk transfer is critical for both initiating and maintaining a woman's milk supply, as lactation is a wonderful balance of supply and demand. There are a number of full-term infant reflexes that are important to the success of breastfeeding including:

  • The rooting reflex, where an infant is able to their mouth open wide and turn their head towards a stimulus, elicited by the mother touching his/her lips or cheek near the corner of the mouth with her finger or breast

  • The sucking reflex, which is usually elicited by the nipple touching the top of the palate deep in an infant’s mouth

Breast Milk Composition


From around the second trimester onwards, a pregnant woman’s body will start to produce colostrum. In fact, some women will experience leakage of colostrum during pregnancy. This thick, sticky, golden yellow liquid (sometimes known as ‘liquid gold’) is the first type of milk that a baby will receive after being born. Only a small volume of colostrum is produced (around 40-50 ml/day), but despite its small quantity it is jam-packed with nutrients and antibodies that are able to sustain a newborn for the first few days of life. Some of the benefits that colostrum conveys to baby include:

  • It is easy to digest and acts as a laxative that helps to expel all of the meconium (a baby’s first poo) relatively quickly

  • This can help play a role in preventing jaundice in a baby’s first few days of life

  • It is rich in immunoglobulins, which helps to kick-start a baby’s own immune system as they adjust to a new, non-sterile environment

  • It helps to seal up a baby’s gut, which prevents foreign substances from crossing the intestinal wall

  • It is rich in minerals and vitamins, with higher quantities of vitamins A, E, and K than mature breast milk

Transitional milk

A few days (usually around 2-4) after giving birth, breast milk will change in composition and a woman will begin to produce more milk than before. This period is often referred to as the milk ‘coming in’, and will probably leave a woman’s breasts feeling fuller and firmer than before. From this time up until day 14 postpartum, a mother will secrete transitional milk, which represents an intermediate between colostrum and mature milk. It will slowly become creamier in colour and texture, and its fat, calorie, and lactose contents will increase. Transitional milk, like colostrum, is full of protective antibodies, probiotic bacteria, and other bioactive ingredients that are beneficial to a baby’s health.

Mature Milk

By the time a baby is 10-14 days old, a mother will be producing mature milk, which is richer in protein, sugar, vitamins, minerals than colostrum and transitional milk. It also contains bioactive components including hormones, growth factors, enzymes, and live cells that support a baby’s healthy growth and development. At this point, a mother will produce approximately 1.5 L of milk per day for a single infant (more if she has twins or triplets). As her infant goes through growth spurts, her milk supply will adjust to accommodate changes in demand. From four weeks onwards, the nutritional content of mature milk will usually remain fairly constant, however the composition of breast milk can still change from day to day or feed to feed depending on the needs of a baby. Mature milk changes from the beginning to the end of a feed. Foremilk is the early milk, which is watery, translucent, and richer in lactose and protein and will quench an infant’s thirst. As a feed progresses, the fat composition of the milk will gradually increase to become hindmilk, which is opaque, creamy, rich in fat, and serves to satisfy an infant’s appetite. This change happens gradually over the course of a feed.

Extra Resources worth exploring

If you would like to better understand the anatomy and physiology behind breastfeeding, check out the videos below when you have some free time.

How do my breasts make milk?

Basic anatomy of the female breast, and the mechanisms of lactation.


[1] Kaplan R. Latch: A Handbook for Breastfeeding with Confidence at Every Stage. USA: Rockridge Press; 2018. 150 p. *

[2] Kadé K. Women's Health Series // Hormone Basics. 2018 Dec 9 [cited 2019 Feb 8]. In: Taste & See Blog [Internet]. Available from: https://www.tasteandseeblog.co.za/journal/womens-health-hormone-basics

[3] The StayWell Company. The Breastfeeding Breast. [cited 2019 Feb 8]. Available from: https://www.fairview.org/patient-education/85662

[4] Geddes DT. The anatomy of the lactating breast: Latest research and clinical implications. Infant. 2007;3(2):59-63. Available from: http://www.infantjournal.co.uk/pdf/inf_014_lbt.pdf

[5] Ramsay DT, Kent JC, Hartmann RL, Hartmann PE. Anatomy of the lactating human breast redefined with ultrasound imaging. J Anatomy. 2005;206:525-34.

[6] BC Open Textbook. Anatomy and Physiology. OpenStax CNX. Available from: https://opentextbc.ca/anatomyandphysiology/chapter/28-6-lactation/

[7] Medela. Breast milk composition: What’s in your breast milk? [cited 2019 Feb 9]. Available from: https://www.medela.co.uk/breastfeeding/mums-journey/breast-milk-composition

[8] Ellis D. Supporting the Breast-feeding Dyad. Can Fam Physician. 1986;32:541-545.


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