The Anatomy and Function of the Breast

By Alison-Heyerdahl and Reviewed by Dr. Melinda Boss, PhD

During pregnancy and after birth, the baby's and mother’s bodies work together seamlessly in a beautiful symphony where both mother and child benefit. Understanding how your breast changes during pregnancy and lactation helps to fully appreciate how breastfeeding works.

Lactation refers to the process that results in your breasts being able to produce milk for your baby. Lactation completes the reproductive cycle and has an important role that supports both your own health and recovery from childbirth, as well as the growth and development of your baby. 

Your breasts start to change as soon as conception occurs (when the egg is fertilized).  Breast changes (for example breast tenderness) are often one of the earliest signs of pregnancy. You may notice changes such as the dark areas around the nipples, known as the areolas, beginning to swell, and the veins along your breast becoming more apparent as blood flow to them increases.  

During pregnancy, a system of milk ducts and small sacs (called alveoli) lined with milk-producing cells called lactocytes form in your breasts. By around the 20 weeks of pregnancy, these lactocytes are able to start producing the first milk secretion (colostrum). Colostrum is produced in small quantities (about 30mL per day) and is available for your baby until about 60 hours after birth.  Colostrum has a very important role that includes protecting your new baby from infection and helping them to pass their first bowel movement.

Fun fact: The cycle of lactation starts when conception occurs (the egg is fertilized) and finishes when a mother’s breasts return to a resting state after the baby completely stops breastfeeding.

Breasts come in a variety of shapes and sizes and the size of a mother’s breasts don’t determine her ability to produce milk. Lactating breasts have a mixture of fat and milk-producing tissue and it is the amount of milk-producing tissue that determines how much milk the breasts can store. The ratio of fat to milk-producing tissue in breasts varies between women. Two women with the same size breasts may be able to store different amounts of milk if one woman has more fat and less milk-producing tissue than the other. Both women can produce enough milk for their babies throughout the day, but the woman with less milk-producing tissue may need to breastfeed more frequently because she can’t store as much milk in her breast (her baby will drink smaller amounts of milk more often).

Once the placenta has been delivered, the levels of a hormone called progesterone in a mother’s body drop dramatically, which triggers the transition from production of small amounts of colostrum to copious quantities of mature milk. 

Milk is stored in the alveoli which are connected to milk ducts.  These ducts carry milk to the nipple. Women have, on average, around nine functional ductal openings per breast. 

The baby's suckle stimulates the pituitary gland to release oxytocin into the bloodstream.  Oxytocin causes the muscles around the milk-filled alveoli to contract, known as the let-down reflex. This let-down reflex usually happens after your baby has been sucking the breast for about two minutes. Most women feel this let-down reflex and what this feels like varies. Mothers have reported feeling sensations such as a tingling, itching, or warmth. The let-down reflex also causes contractions in the uterus, which help mothers recover from childbirth and prevent too much blood loss. These contractions are commonly known as “after pains” and may be felt by some mothers as cramping in the abdomen or lower back pain. After pains are more common in women who have had more than one baby and usually fade within the first 2 weeks after birth.

For mothers to be able to produce enough milk for their baby, it is important that the baby breastfeeds frequently and effectively from birth.

Removal of milk from the breast (by breastfeeding or milk expression) sends the message to a mother’s system that more milk needs to be produced. Exclusively breastfed babies need to feed about 8-12 times in 24 hours.

Interesting Fact: Motherhood has long lasting positive effects on the brain in rodents. From an evolutionary point of view, this helped mothers care for their babies, giving them a competitive advantage for survival.