First, consider just the capacity.

Unfortunately, there is no clinically easy way to determine the milk-making capacity of a particular mother. Since the milk-making capacity of the breast increases over the first month, full capacity is not always apparent until week 4. Also, if mother-baby pairs do not regulate together early on, the capacity of the breasts does not always develop to its full potential. If the baby is in the NICU, for example, and the mother does not pump or hand express, she may miss an important window to maximize her milk-making potential. When it comes to capacity, every day during the first month matters. Interestingly, each breast has its own capacity, independent of the other breast. So if one breast has damage or mastitis, the other can continue to produce milk.

In the past, researchers attempted to determine capacity by test-weighing the infant, mother, or both. But these measures really tested infants’ transfer ability, not mothers’ milk-making capacity (Daly and Hartmann 1995a, b). Also, Arthur et al. (1987) found that evaporative water loss (EWL) can result in an overestimate of the amount of milk transferred by an infant. In test-weighing mothers, EWL accounted for a wide range of the weight difference (3–94 %). In test-weighing infants, 3–55 % of the weight change was attributable to EWL.

Pumping the breast is one direct measure of capacity, but a single pumping session is not reflective of the full capacity. The volume of stored milk varies throughout the day, so 24-hour collective pumping is more reflective of the breast’s abilities than one pumping session. For example, Benson (2001) found that during the first week, babies nurse most frequently from 3:00 am to 9:00 am, which correlates with increased prolactin levels in mothers, which in turn creates more overnight milk (Cregan et al. 2002). Additionally, different pump vacuum pressures and timing of emptying result in different degrees of removal. The pump also cannot extract milk from the breast as well as an efficiently suckling infant.

Dr. Peter Hartmann and the Human Lactation Research Group at the University of Western Australia developed a research tool for indirectly measuring milk capacity at a particular feeding, called the Computerized Breast Measurement (CBM) system. It utilizes hardware and software to measure the distortion of light patterns by glandular breast tissue to measure changes in breast volume. The change in breast volume before and after breastfeeding was then compared to the amount of milk removed by the infant as determined by test-weighing. They found a close relationship between the removal of milk by the infant and the change in breast volume on CBM in infants who are successfully nursing.

The CBM system does not determine absolute breast volume, but rather measures a relative volume of the part of the breast that produces the most milk. They did a series of studies using this system to study how milk production is regulated. It is important to note, however, that these studies were done on women who were already successfully nursing (during months 1–6). This means that patterns had already been established, and the mother and baby had already regulated to one another. The results do not pertain to the first month of nursing, when the patterns are being set. Further, the sample sizes were quite small (n = 4–11). However limited, these studies laid the groundwork for much of our understanding of global and local regulation of milk supply.

The first month of milk production is the most critical. This is because global hormonal regulation in the mother lays the groundwork for future milk production. After the first month, it is possible to increase the supply, but there is a limit to the degree of increase, and it is more difficult to accomplish. De Carvalho et al. (1983) discovered that the more frequently babies nursed before the 35th day, the more milk was transferred and the more weight they gained. Seems completely logical. However, after day 35, there was no increased weight gain or milk intake from more frequent feedings. Let’s walk through why this is so.

The first few days after birth, colostrum is produced by the breast because progesterone levels drop and prolactin peaks. Touch receptors on the breast, stimulated by infant suckling, increase the number of prolactin receptors at the breast. This only happens for the first few days, then that window for more prolactin receptors closes. Touch receptors also stimulate release of oxytocin, which results in myoepithelial cell contraction and the milk ejection reflex. This reflex happens several times during each nursing session and is responsible for fully emptying the breasts. When the baby nurses, prolactin is released 30 min afterwards, to stimulate lactocytes to produce more milk for the next feeding.

At day 3–7, colostrum converts to transitional milk, and milk volume increases. From week 1–2 transitional milk changes to mature milk and circulating prolactin levels begin to drop. At this point, the lactiferous ducts are mostly filled with foremilk, which is watery and has a higher concentration of lactose. The alveoli contain more fatty milk, or hindmilk. So the heavier milk is the last to leave the breasts. Oxytocin continues to work as it has, stimulating the myoepithelial cells, which surround the alveoli to squeeze out the hindmilk and push the foremilk forward and out. When the hindmilk is finally removed, the baby gets full. By 1 month, the volume of breast milk reaches its peak and stays consistent throughout the remainder of breastfeeding. This is because the volume required by the infant also stays constant.

By 6–10 weeks, circulating prolactin is back down to near pre-pregnancy levels. Having lower amounts of circulating prolactin means that the breasts must have a lot of prolactin receptors so they remain very sensitive to the hormone’s effects as breastfeeding continues (Neville et al. 1988). In this way, tiny surges in prolactin in month 2 stimulate the same amount of milk production as larger prolactin surges in week 2, simply because there are so many more receptors in the lactocytes of the breast. Again, it cannot be stressed enough that this is why breastfeeding problems need to be addressed as quickly as possible, preferably in the first 2 weeks, so that milk capacity can be optimized.

This system is fine-tuned by the fullness of the breast. Bound prolactin causes an increase in milk production, resulting in fuller breasts. As the breasts fill with milk, the prolactin receptors stretch, changing shape so prolactin no longer binds and milk production slows (Cregan et al. 2002).

Once the groundwork has been established after the first several weeks, the regulation of milk production transitions from global to local control. While global controls may still be active in the background, the milk supply becomes more dependent on the infant’s milk removal than the mother’s ability to make milk (Daly and Hartmann 1995a, b, part 1 and part 2). In other words, the breasts continue to provide as much milk as the infant needs by refilling in response to milk removal. But there are parameters of milk removal that must be considered: The frequency, degree of emptying, and duration of milk removal ultimately determine milk production and regulation.

There is an optimal pattern that maximizes the milk production available to the infant.

Frequency of nursing is important to a certain degree. Daly et al. (1996) showed that milk synthesis remained constant if the breasts were emptied every 1–6 h. After 6–18 h, milk production was reduced compared to when the breasts were emptied more frequently. This correlates with the mother’s milk-making capacity, because if she can store a lot of milk, then less milk will be made if the milk is not removed. If the breasts are not emptied in 24 h, the tight junctions between cells break down and the glandular tissues in the breast involute. Breasts go back to their pre-pregnancy status and milk production eventually ceases.

Many studies have established that the milk supply responds not only to the frequency of nursing, but even more so to the degree of emptying (De Carvalho et al. 1982, 1983; Butte et al. 1984, 1985; Dewey and Lönnerdal 1986; Dewey et al. 1991; Pinilla and Birch 1993). In other words, the more thoroughly the breast is emptied, the more milk is produced.