During pregnancy the breasts develop considerably as any pregnant woman will know! Fat is deposited around the glandular part of the breasts leading to an increase in the size of the breasts, and oestrogen leads to a further increase in the size by increasing the number of the ducts. Progesterone lays a further hand by increasing the number of alveoli. Human placental lactogen (secreted by the placenta) further stimulates alveolar development and may be involved in actual milk production (by making casein, lactalbumin and lactoglobulin) by the alveolar cells.

So why don’t we have milk when pregnant (other than the gods looked kindly upon us uncomfortable beings and thought… “nah, we couldn’t do that to them too…”)?

Lactation (making milk) is prevented (inhibited) during pregnancy, although the levels of human prolactin rise through pregnancy. The reason for this is the high levels of oestrogen occupy binding sites on the alveoli which prevent them from responding to the other vital hormone in milk production which is human Prolactin. In late pregnancy, the breasts secrete a thickish, yellowish fluid called colostrum, which is laden with immune antibodies and the best thing ever for newborns! The production of colostrum increases after the birth until it is replaced with breast milk.

After childbirth

The level of oestrogen falls rapidly in the 48 hours after childbirth, which allows the circulating hormone that wasn’t allowed attach pre-birth to now act on the alveoli and to start lactation (milk!),and to keep this going. Lactation (milk production) is encouraged by frequent sucking, as this reflexly causes the pituitary gland (in the brain – see diagram) to secrete (release) Human prolactin.

By day 2 or 3, the alveolar cells have been stimulated enough to release milk, which is thin and bluish in colour, and fill the breasts to rather different looking ones we were previously used to!

Initially the milk distends the alveoli and small ducts causing the breasts to become large and tender, expanding the now visible veins beneath the skin. Milk ducts can no be felt beneath the skin. All these dramatic changes however do settle down markedly we promise!! And no it’s not lattes you’ll be producing though they certainly feel hot enough! Which brings us to….

The milk ejection reflex

The Prolactin Reflex

2. (Short arrow) The pituitary gland releases prolactin into the blood
3. (Breast) This causes the alveolar cells to secrete milk and swellsthe alveoli
1. (Long arrow) Nerve impulses from sucking go to brain

The Milk Ejection Reflex

1. (Long arrow) Nerve impulses from sucking go to the brain
2. (Short arow) The pituitary gland releases oxytocin into the bloodstream
3. (Breast) This causes muscles around the alveoli in the breast to squeez milk to the nipple

The milk filling and distending the alveoli is not available to the nursing infant until the myoepithelial cells (see diagram below) – contract and squeeze out the milk in response to the milk “let down” reflex. This reflex is started by even more sucking, and is made possible though the hypothalamus (another brain part) and pituitary gland which releases oxytocin (a hormone) into the bloodstream. This hormone causes contractions of cells within the breast and milk is ejected from the alveoli and small ducts to flow into “reservoirs” within the breast. Both negative and positive emotional factors affect this let down, so it is VITAL that a mother feels confident in her ability to breast feed, and should get very strong encouragement from those around her at this early stage.

The most effective way of maintaining lactation is regular sucking/ nursing, so that both the prolactin and the milk ejection reflexes are initiated frequently.

Mothers need to know how to make use of their babies’ natural reflexes. The baby’s entire body should be turned towards the mum with the neck in a relaxed position. (tummy to tummy). Brushing baby’s lips against the nipple elicits the “rooting reflex” (Prechtl 1958). The baby’s mouth will gape open. The wider the gape, the easier the latch. See “How to do it” (Link)

Further information and links:

Medical:

Human Milk and Lactation. Carol Wagner, Professor of Paeds.
Complete Info on the lactating breast, biochemistry of milk, lactogenesis…

Non-medical:

Anatomy of the breast & how it makes milk. Marie Davis IBCLC

How the breasts make and deliver milk. Dr Sears.

Finish the breast first. By Melissa Vickers.
Feeding patterns, baby led feeding.