What happens inside your breasts to make your milk come in

How milk is produced?

Progesterone, oestrogen, and prolactin levels rise in your blood stream as your pregnancy progresses. The high levels of progesterone washing through your blood stream inhibit the activity of prolactin, which is a main milk-making hormone.

But once your placenta is delivered, progesterone levels tank and unleash your prolactin, causing your milk to come in - something women usually feel within about 72 hours. These first few days after you've given birth are the only time when your milk production is predominantly controlled by hormones from the brain and blood stream.

When your milk lets down

Milk flows from your breast when oxytocin is released from the pituitary gland in your brain, in response to your baby's suckling. Oxytocin stimulates contractions of the microscopic layer of smooth muscle cells around your milk glands, known as myoepithelial cells. This results in a milk ejection reflex or letdown.

Many women can't feel their letdown, or feel it only sometimes - and anyway, your baby draws milk from the glands and ducts even when there isn't a letdown, just in smaller volumes. The more milk that is taken from your breast, the more your milk supply is stimulated in that breast.

Your milk comes in

We can measure the change from colostrum into transitional milk 30 to 40 hours after you've given birth. There is an abrupt increase in the rate of milk protein and lactose synthesis at about 30 hours after the birth.

These changes are measurable a day or more before you have the feeling that your milk has come in. Women often feel their milk coming in around 72 hours after you've given birth, though it can take up to five days after the birth (and this is normal).

Your lactocytes draw in the glucose which travels in rich network of capillaries wrapped around the alveoli, and also galactose, combining these two molecules to make lactose. Lactose is secreted in large amounts into the lumen of the glands, drawing water with it by defusion, and spilling out into the ducts. Lactose is a key ingredient in your transitional milk.

Fat droplets are secreted too. The white cell population falls by four-fifths.

Only a few of the components of your milk are directly derived from your blood stream. The most important substances, such as fat, protein, and lactose are synthesized in the lactating cell. It is possible that some lactocytes are specialized, some for instance producing more fat, others more lactose.

Sodium levels drop, and even though milk protein is now being synthesised by the lactocytes, relative protein concentration in transitional milk is a lot less than colostrum. Leucocyte populations fall by four-fifths between colostrum and mature milk

Milk production is controlled by milk removal

Once your milk has come in, your milk glands are no longer as sensitive to prolactin. From about 30 or 40 hours after the birth, it is the removal of milk which determines how much milk your breasts will make for your baby.

Up to eleven breastfeeds in the first 24 hours is shown in the research to result in more milk on the fifth day (compared to say six offers in the first 24 hours). Milk production at two weeks is predictive of milk production at six weeks. This is why frequent flexible breastfeeds or milk removal from the birth onwards is so important for the establishment of a supply of milk which meets your baby's caloric needs.

Selected references

Witkowska-Zimny M, Kaminska-El-Hassan E. Cells of human milk. Cellular and Molecular Biology Letters. 2017;22(11):doi:101186/s111658-101017-100042-101184.

Yamauchi Y, Yamanouchi I. Breast-feeding frequency during the first 24 hours after birth in full-time neonates. Pediatrics. 1990;86(2):171-175.

Hill PD, Aldag JC, Chatterton RT, Zinaman M. Primary and secondary mediators' influence on milk output in lactating mothers of preterm and term infants. Journal of Human Lactation. 2005;21(2):138-150.