Milk production doesn't happen in one single stage; it's a process that evolves. Initially, during Lactogenesis II (which typically occurs 2-5 days after birth), the system is under endocrine, or hormonal, control. The delivery of the placenta causes a sharp drop in progesterone, which gives prolactin the green light to kickstart copious milk production. At this point, the body is making milk regardless of whether the baby is nursing effectively or not. It's an automatic, hormonally-driven event.

Around day 9, a critical transition happens. The system shifts to autocrine, or local, control. This is Lactogenesis III, the long-term phase of milk production. From this point forward, milk supply is no longer primarily driven by systemic hormones but by the principle of supply and demand within each breast. Each breast begins to regulate its own production rate based on how frequently and how completely it is drained. This is where the local feedback mechanisms become the star of the show.

The key to local control is a small whey protein present in the milk called Feedback Inhibitor of Lactation (FIL). Think of FIL as a local manager on the factory floor of each breast. When milk is removed, FIL is removed along with it. A low concentration of FIL sends a clear signal to the milk-producing cells, the alveoli, to ramp up production. The factory is open for business.

Conversely, if milk is left to accumulate in the breast for longer periods, the concentration of FIL increases. This high concentration signals the alveoli to slow down production. If milk isn't being removed, the manager tells the workers to take a break. This is why frequent and effective milk removal is the cornerstone of maintaining a robust supply. It keeps the level of this inhibitory peptide low, ensuring the 'make more milk' signal is always on.

It’s also important to consider breast storage capacity. This isn't about breast size, but rather the amount of milk a breast can hold between feedings before it becomes full. A mother with a smaller storage capacity may need to nurse more frequently to keep FIL levels low and maintain her supply, while a mother with a larger storage capacity might be able to go longer between feeds without a drop in production. It’s a highly individualised aspect of lactation.

Even with local control in charge, hormones still play a vital role. The two main players are prolactin and oxytocin, which work in tandem. When a baby nurses, nerve endings in the nipple send signals to the brain.

One set of signals goes to the anterior pituitary gland, telling it to release prolactin into the bloodstream. Prolactin acts as the production signal, instructing the alveolar cells to make more milk to replace what was just removed. This ensures the breast is ready for the next feeding.

Simultaneously, another signal travels to the posterior pituitary gland, triggering the release of oxytocin. This hormone causes the tiny muscle cells (myoepithelial cells) wrapped around the alveoli to contract. This contraction squeezes the milk out of the alveoli and into the milk ducts, pushing it towards the nipple. This process is known as the milk ejection reflex, or let-down. Some mothers feel this as a tingling or pins-and-needles sensation, while others feel nothing at all.

Understanding this dual system of local, demand-driven control and hormonal reflexes is key. It clarifies why frequent and effective milk removal is more critical for supply than any other factor. The body is designed to respond directly to the baby's needs, creating a dynamic and responsive feeding relationship.

Let's review these physiological processes before moving on.