How do breasts make milk?

Colostrum and breast milk production is a series of four complex stages that start soon after the mother becomes pregnant and is guided by the mother’s hormones and on-going milk removal from the breast. Milk production continues until weaning occurs. Colostrum and milk are made by cells lining the milk sacs in the breast. Those cells draw nutrients from the mother’s blood. For maximum milk production, milk must regularly be removed from the breast beginning within one hour after delivery.

A) Describing lactogenesis

Colostrum and breast milk are incredibly complex biofluids that contain a collection of components that work together to ensure the optimum nutrition and growth of babies. They are unique to each mother and change over time.

Lactogenesis is the process the breast undergoes to start and continue secreting, or releasing special liquids for the baby’s use. The cells lining the milk sacs (alveoli) in the breast use nutrients drawn from the mother’s blood to create these liquids. The process is controlled by many hormones and by colostrum or milk being removed from the breast.


  • Starts during pregnancy with the production of colostrum.
  • Continues with the production of transitional and mature breast milk.
  • Ends with the production of weaning milk when the baby or mother chooses to wean.

Lactogenesis is divided into four stages based on the type of milk made. These are usually written using latin numbers (I, II, III, IV). Please see the following table for more information.

Table: The Stages of Lactogenesis

Colostrum is clear and straw-coloured, transitional milk looks very rich and creamy, and mature milk will look thinner and slightly more blue over time. These are all normal changes.

B) Stages of lactogenesis

1) Lactogenesis I

Lactogenesis I is when the breasts make colostrum. This starts as early as 16 weeks and as late as 22 weeks into pregnancy (Lawrence 2016).

2) Lactogenesis II

Lactogenesis II is when the breasts change from making colostrum to making transitional milk.

The placenta makes the hormone progesterone, which blocks the action of prolactin, one of the hormones needed to start milk production. When the mother delivers the placenta, the level of progesterone quickly falls triggering lactogenesis II (Ostrom 1990).

Mothers will start making milk 24 to 96 hours (average 60 hours) after delivery. This is also called milk “coming in” or secretory activation. The amount of transitional milk is much larger than that of colostrum and as a result most mothers will notice definite changes in their breasts as the milk comes in.

The first feed at the breast should be within one hour after birth. If breastfeeding is not possible, mothers should express within one hour after birth. From then, milk must regularly be removed from the breast for it to maximize the amount of milk made. This can be done by effective breastfeeding or regular, effective expressing.

The amount of milk made over the long term depends very much on establishing milk production within the first two weeks. During this time, milk cells lining the alveoli and the number of prolactin receptors on these cells increase in number (Meier 2012).

3) Lactogenesis III

Lactogenesis III is when the breast makes steady amounts of mature milk. This starts in the third week after birth and continues until weaning starts. On-going milk production needs regular milk removal. 

4) Lactogenesis IV 

Lactogenesis IV is when the breast stops making milk as weaning happens. Weaning milk has differences in some of the nutrients compared to mature milk. Weaning can proceed at different rates and this will determine how long milk continues to be produced by the breast.


Lawrence RA and Lawrence RM. Breastfeeding: A Guide for the Medical Profession. Elsevier; 2016
Meier PP,  Engstrom JL, Janes JE, et al. Breast pump suction patterns that mimic the human infant during breastfeeding: greater milk output in less time spent pumping for breast pump-dependent mothers with premature infants. J Perinatol 2012 Feb;32(2):103-10
Ostrom, KM. A review of the hormone prolactin during lactation. Prog Food Nutr Sci. 1990;14(1):1-43