Anatomy and Physiology

By Dr Clare Macdonald, General Practitioner

Breast Anatomy

Breast development begins in puberty and cyclical hormonal changes promote development and proliferation of adipose tissue and the ductal system which will ultimately be required for lactation. The ductal system is largely dormant until pregnancy; hormones of pregnancy trigger crucial changes in the breasts:

  • Oestrogen promotes the development of milk ducts (ultimately responsible for milk storage/delivery)
  • Progesterone stimulates alveolar-lobular formation (ultimately responsible for milk production)

Multiple changes to breast appearance are entirely normal during pregnancy:

  • Increase in size due to alveolar-lobular formation
  • Appearance of Montgomery Glands – small, raised bumps on the areola, responsible for nipple lubrication and antibacterial properties
  • Darkening of the areola
  • Veins become more prominent

Lactation Physiology

In the second half of pregnancy prolactin triggers the production of colostrum (Lactogenesis I) but levels of oestrogen and progesterone are high enough to prevent significant lactation. Delivery of the placenta triggers a sudden drop in these hormones which, leaves the very high levels of prolactin levels free to trigger the onset of copious lactation (Lactogenesis II). From this point, oxytocin and prolactin are the hormones responsible for maintaining milk production and release.

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  • Nipple stimulation by a suckling infant or breast pump causes a surge in oxytocin, causing a milk ejection reflex (let down) and thus movement of milk from the alveoli into the milk ducts.
  • The resultant emptying of the alveoli enables prolactin to activate prolactin receptor sites on the walls of lactocytes (milk producing cells of the alveoli) triggering further milk production.  This mechanism ensures that milk synthesis meets the infant’s requirements; the more frequent nipple stimulation and let down, the more milk is produced.
  • Early milk production relies on priming of lactocytes by stimulation of prolactin receptor sites and ongoing milk supply is controlled by the degree of breast emptying and levels of Feedback Inhibitor of Lactation (Lactogenesis III).
  • The whey protein Feedback Inhibitor of Lactation (FIL) increases in concentration in each breast if milk is not removed and the alveoli become distended. FIL inhibits milk synthesis via autocrine control and antagonises the effect of prolactin.

Breastmilk Composition

Human milk consists of water, triglycerides, fatty acids, lactose, proteins, vitamins, calcium, phosphate as well as living B and T lymphocytes, neutrophils, macrophages, stem cells, enzymes, hormones, growth factors, oligosaccharides and many other components. Colostrum contains higher concentrations of the immune components, sodium and chloride but less lactose and triglycerides.

Additionally the immunological benefits of breastfeeding to the infant are achieved through various mechanisms in addition to the presence of immune cells in breastmilk. Lactoferrin and Bifidus factor promote gut protection and other components aid gut and immune development. IgA immunoglobulins present will be specific to the pathogens in highest concentration on and around the mother, and therefore provide protection against the infections to which the infant is most at risk.  This is of particular importance given the infant’s immature immune system.

Infographic Final A1
With thanks to Human Milk CIC

The dynamic nature of breastmilk means that an individual woman’s milk changes in composition both over the course of each day and throughout the time she is breastfeeding as the infant grows. Despite the wide global differences in maternal nutrition, breastmilk from different women remains highly comparable. Infant-specific differences in milk composition are well documented, for example the breastmilk of mothers of preterm infants typically contains higher concentrations of protein and this is proportional to the degree of prematurity.

The milk towards the end of a feed can contain two to three times the concentration of fat than the first milk of a feed. The historically used terms ‘foremilk’ and ‘hindmilk’ do not convey the gradual change in fat content of the milk throughout the feed. This is why it is important to avoid switching to the second breast before the infant has ‘drained’ the first breast as much as possible. Additionally, the fat content changes over the course of the day.

This is milk expressed by a mother after a day at work without feeding or pumping. The milk on the left is the first milk expressed, and the milk on the right is from the end of the period of expressing.  The proportion of fat in the early yield is visibly significantly lower than in the final milk sample.

Further Information

KellyMom: How Does Milk Production Work?

First Steps Nutrition Trust: Breastmilk and Breastfeeding, a Simple Guide Scroll down to the blue and pink guide.


Human Physiology From Cells to Systems, Lauralee Sherwood, Brooks/Cole (4th Edition 2001) ISBN 0-534-56826-2

Field C.J. The Immunological Components of Human Milk and Their Effect on Immune Development in Infants. Journal of Nutrition. 2005;135(1):1-4

Unicef UK Baby Friendly Initiative: Three-day course in Breastfeeding Management Participant’s handbook (November 2008)

Ballard O, Morrow AL. Human Milk Composition: Nutrients and Bioactive Factors. Pediatric Clinics of North America. 2013;60(1):49-74.

Published April 2017, Updated October 5th 2019