Understanding breast milk composition

Breast milk production

During pregnancy, a mother’s body prepares for breast milk production. The hormones estrogen, progesterone, and prolactin work together to stimulate the milk-producing glands (alveoli) and cause breast enlargement. In the second and third trimesters, rising prolactin levels initiate colostrum production. However, high progesterone levels maintain colostrum without triggering mature milk production.

After the baby and placenta are delivered, estrogen and progesterone levels drop while prolactin rises, signaling the body to start mature milk production. It takes the body 10 to 15 days to produce mature milk. The baby relies on colostrum and transitional milk until mature milk is produced.

Stages of breast milk

Breast milk composition adapts to meet a baby’s changing needs as they grow.

• Colostrum: Referred to as "liquid gold," has a yellow hue and a thick consistency. It is the first milk produced during late pregnancy and the first few days after birth. Colostrum is highly concentrated in protein, antibodies, and immunoglobulins, to support newborn immunity and digestion. It’s also lower in lactose for easier digestion. A newborn’s stomach is the size of a marble, so they rely on small, frequent feedings of highly nutrient-dense colostrum in the first few days after birth.
• Transitional milk: The milk phase between colostrum and mature milk that the baby drinks for 2 to 4 days after birth.
• Mature milk: Cream-colored milk that becomes fully established around two weeks postpartum. It is higher in fluid, fat, and carbohydrates, and provides ongoing nourishment for the duration of breastfeeding.

Foremilk and hindmilk

• Foremilk: The milk released at the start of a feeding or pumping session. It’s higher in water and carbohydrates and lower in fat and calories.
• Hindmilk: The milk released at the end of a feeding or pumping session. It is richer in fat and calories.

Breastfeeding hormones: Prolactin and oxytocin

Breast milk production is influenced by hormones. These hormones must be balanced for successful breastfeeding.  

• Prolactin is the primary hormone responsible for milk production.
• Oxytocin is the hormone responsible for releasing breast milk, or the let-down reflex.

Oxytocin, also referred to as the “feel-good” hormone, promotes relaxation, which can influence breast milk production. Stress and anxiety can suppress oxytocin levels, resulting in poor milk release or let-down. To encourage milk flow, mothers are often advised to:

• Gently massage their breasts
• Practice skin-to-skin contact with their baby
• Create a calm and relaxing environment before breastfeeding or pumping

To improve prolactin levels and support milk production, mothers are encouraged to smell their baby’s clothing or watch baby videos to stimulate milk supply.

Breast milk composition

• Water: 87 to 88%.
• Carbohydrates: 7% (lactose and oligosaccharides).
• Fat: 3.8% (triglycerides).
• Proteins: 1% (casein and whey).
• Fat soluble vitamins: Vitamins A,D,E, and K.
• Water soluble vitamins: B vitamins and vitamin C (amount depends on maternal diet and supplementation).
• Minerals: Calcium and phosphorus.
• Other: Antibodies, human milk oligosaccharides (HMOs), immunoglobulins, and lactoferrin.

Breast milk continues to adapt as the baby grows. At 6 months postpartum, breast milk contains higher levels of immune-boosting components to support the baby’s developing immune system. At this stage, it also becomes slightly less calorie-dense, as babies begin eating solid foods and their growth rate slows compared to the first six months.

What's the difference between breast milk and animal milk?

Breast milk is higher in lactose and fat, but lower in protein and minerals to make it easier for developing kidneys to process. Breast milk also has higher amounts of whey protein, which makes it easier to digest. Cow’s milk has more casein and does not contain HMOs or antibodies.

When to consult a lactation specialist

If you have questions about breastfeeding or have trouble with latching or positioning, consult your local or Summer Health lactation specialist for additional support.