How Your Body Knows
So how do the breasts figure out how much milk to make? In the beginning, some mothers seem to initially make enough for two babies and are overflowing with milk, though quite often mothers start off lower and their production gradually increases to meet baby’s needs.(8) Your eventual milk production level is determined by the feeding interactions between you and your baby (or breast pump) and how they influence your hormones.
In the next several weeks after birth, average (or baseline) prolactin levels decrease until they reach a lower plateau. At the same time, receptors for prolactin are multiplying. The development of prolactin receptors in the breast is believed to be related to early frequent suckling stimulation and milk removal: the more often baby breastfeeds in the first days and weeks after birth, the more receptors are developed. (9), (10), (11)
During the time when prolactin levels are dropping and its receptors are being established, the milk making process transitions from a largely hormone-driven (endocrine) system to one that is controlled more locally in the breast (autocrine). Under autocrine control, the breast responds to baby’s demand by adjusting the speed of milk production upward or downward according to the amount of milk removed. The more milk that is stored in the breast, the slower milk is produced; the emptier the breast, the faster the production of milk. The breast is very sensitive and responsive to the degree of fullness. As the breast fills up, concentrations of a whey protein called the feedback inhibitor of lactation (FIL) rise and trigger a cut-back in the rate of milk production, much like we would slow the water down in a bathtub that is filling up too quickly. Concentrations of FIL are low when the breast is empty, allowing the breast to produce milk faster just as we would turn the water faucet on high when a bathtub is empty and we want to fill it up.
Every time baby suckles at the breast, nerves in the nipple and areola are stimulated, sending messages along nerve pathways to the brain saying that milk is needed. The pituitary responds by releasing a surge of prolactin, causing a temporary increase in prolactin levels to encourage continued re-fueling of the breast. As long as milk is being removed and is not building up in the breast, more milk will be made. More new milk is made when the breast is emptier, and less is made when the breast is fuller.
The goal of the autocrine process is to fine-tune your milk production to meet baby’s actual needs. Like the marketing research department in a factory, your body spends the first few weeks after baby is born determining how big of a factory is needed and how fast it must work to meet baby’s milk needs.(12) In essence, all the experiences of how often baby nurses and how much milk he removes is part of the body’s “market research phase” as it calibrates your milk production. This period is critical in laying the blueprint for a milk making factory that will meet baby’s needs until he no longer needs it.