The making of milk

There are two parts to the making of milk.

• During pregnancy your milk-making glands and their ducts undergo a long preparation.The preparation known as secretory differentiation begins at about twenty weeks into the pregnancy. But your new milk glands don't actually secrete milk until your baby is born.

• Colostrum isn't secreted by your milk-making cells. It's produced during secretory differentiation, from about 20 weeks into your pregnancy, and can often (but not always) be expressed from 36 weeks into your pregnancy.

• Your breasts ramp up into production over the first two or three days after baby's birth, until your milk comes in. Milk coming in is formally known as secretory activation.

Your breasts prepare to make milk during pregnancy

Women usually feel breast changes in pregnancy but some women who breastfeed successfully don't feel breast changes in pregnancy

Your breasts may not change size at all in pregnancy, regardless of all of this amazing preparatory activity. Or they may increase in size a great deal! Women who go on to successfully breastfeeding throughout the first year of their baby’s life have highly variable change in breast size in their pregnancies. Normal is a very diverse condition!

Again because normal is such a diverse condition, women whose breasts are perfectly normal and capable of producing all the milk their baby needs may also not experience any of the following other signs.

• Darkening of nipple and areola

• Enlargement of areola

• Enlarged sebaceous glands on the areola

• Increased network of veins visible under the skin.

What is happening inside your breasts during pregnancy?

As the first half of your pregnancy passes and your belly swells, the cells of your body are bathed in large amounts of hormones oestrogen and progesterone.

• Oestrogen switches on the production of prolactin, and as a result of oestrogen and prolactin, your breasts' previously undeveloped epithelial cells which line the ducts proliferate, and side-branch. The ducts also blossom at their ends into lactocytes and the generation of thousands of little glands, lined by these lactocytes. An intense growth of glandular tissue occurs.

• Pregnancy is a phase of intense cell proliferation. As the number of alveoli increase and the ducts branch and extend, they generate mechanical forces and extend into your breasts' fatty tissue. The ducts lengthen and flourish into a rich profusion of side-branches, placing mechanical pressures on the surrounding breast tissue. The cells lining the ducts blossom into lactocytes and thousands of tiny alveoli or milk glands are created. As the number of alveoli increase and the ducts branch and extends, your glandular tissue presses into the fatty tissue.

• By mid-pregnancy, blood flow to your breasts has doubled, and your lactocytes start to secrete colostrum, a dense early 'milk', into the lumen.

• Branching of ducts continues throughout the second half of pregnancy, but the blood flow to your breasts remains stable throughout the second half of pregnancy and lactation.

• Ductal epithelial and myoepithelial cells include ancestral cells, long-lived stem cells which have extensive renewal capacities at different stages of mammary gland development.

Lactocytes join together at what we call tight junctions. Under the lactocytes, there is a layer of star-shaped smooth muscles cells, which contract when they are bathed in oxytocin. A dense membrane wraps around the whole gland or alveolus. During pregnancy, the tight junctions are loose and the basement membrane is permeable. This allows protective antibodies and white cells to travel easily out of the networks of capillaries and lymph vessels which nestle up close and wrap around your milk glands. The antibodies and white cells travel easily through the basement membranes and tight junctions, into the lumen of the milk glands. These and bacteria and other protective molecules gather with increasing concentration inside the lumens and milk ducts as your pregnancy progresses, to form colostrum.

Your breasts begin to make milk in the first few days after your baby is born

The timing of the milk coming in, known as secretory activation, is highly variable, occurring between days two to five in normal women. You can find out about your milk coming in here and here.

Each pregnancy increases the amount of glandular tissue and ductal branching that is in your breasts, which increases your milk-producing capacity next time around.

Most women have the physiological capacity to produce all the milk her firstborn baby needs, as long as

• Baby has frequent and flexible access to the breath from birth, and

• You know how to protect yourself (or your breastfeeding support professional is able to protect you) from pain and damage!

Our complex birth environments often interfere with early experiences, but women and their babies are incredibly resilient as long as you have a few strategies to help things get back on track.

You can find out about the genetic or medical reasons why some women's breasts don't make enough milk to meet your baby's caloric needs without supplementation, no matter what they try, here.

Possums Breastfeeding & Lactation currently doesn't address issues related to breastfeeding and lactation when your baby is born preterm. This is under development.

Selected references

Geddes DT. Ultrasound imaging of the lactating breast: methodology and application. International Breastfeeding Journal. 2009;4:doi:10.1186/1746-4358-1184-1184.

Cox DB, Kent JC, Casey TM, Owens RA, Hartmann PE. Breast growth and the urinary excretion of lactose during human pregnancy and early lactation: endocrine relationships. Experimental Physiology. 1999;84:421-434.

Kent JC, Gardener H, Geddes DT. Breastmilk production in the first 4 weeks after birth of term infants. Nutrients. 2016;8 (765):doi:10.3390/nu8120756.

Pang WW, Hartmann PE. Initiation of human lactation: secretory differentiation and secretory activation. Journal of Mammary Gland Biology and Neoplasia. 2007;12:211-221.