The Milk Ejection Reflex (MER), commonly known as “letdown,” is the biological mechanism that causes milk to flow from the breast during feeding. The experience is highly individual, with some feeling a strong tingling sensation and others noticing no physical change. The reflex, where milk begins to flow rapidly, typically occurs within the first minute or two of stimulation. This strong initial flow associated with this reflex usually lasts for a matter of seconds up to a few minutes.
The Physiology of Milk Ejection
The process of letdown is a neuro-hormonal reflex triggered by the stimulation of the nerves in the nipple and areola, usually from an infant’s suckling. These nerve signals travel to the hypothalamus in the brain, which then prompts the release of the hormone oxytocin from the posterior pituitary gland.
Once released into the bloodstream, oxytocin travels to the breast tissue, targeting specialized cells surrounding the alveoli. The alveoli are the small, grape-like sacs where milk is produced and stored. Oxytocin causes the myoepithelial cells, which act like tiny muscles, to contract. This muscular contraction squeezes the milk from the alveoli, pushing it down the narrow milk ducts toward the nipple. This oxytocin-driven contraction ensures the milk produced is made available to the infant.
Factors Influencing Letdown Speed and Duration
The speed at which the letdown reflex begins and the perceived duration of the fast milk flow can be influenced by a combination of internal and external factors. The reflex can become a conditioned response, meaning it can be triggered by sensory cues other than direct nipple stimulation. For instance, hearing a baby cry, even if it is not one’s own, or simply thinking about the infant can sometimes initiate the reflex.
Factors that increase comfort and relaxation tend to encourage a prompt letdown by supporting the release of oxytocin. These include environmental elements like finding a private, quiet space or applying warmth to the breasts. Conversely, factors that trigger the body’s stress response can inhibit the reflex.
Maternal stress, anxiety, or pain can introduce stress hormones, such as cortisol and adrenaline, into the system. These hormones interfere with the release of oxytocin, potentially delaying or slowing the onset of the milk ejection reflex. Fatigue and the use of certain substances like caffeine or alcohol may also slow down the reflex. A proper, deep latch by the infant is also a physical factor that significantly affects the reflex.
How Many Letdowns Occur per Feeding
Milk removal requires multiple milk ejection reflexes throughout the session, not just a single event at the start. Most individuals experience between two and four significant letdowns during a single feeding or pumping session. The initial letdown is often the strongest and the one most likely to be felt as a tingling or sudden fullness.
After the initial surge of milk slows down, the infant’s sucking pattern will typically shift back to a faster, shallower suck. This change in action stimulates the nerves again, prompting the brain to release another pulse of oxytocin. This triggers a subsequent, smaller letdown, which maintains the flow of milk.
The later letdowns are frequently not noticeable to the breastfeeding individual, even if the first one was. The baby’s pattern of swallowing changing from rapid gulps to slower, deeper swallows is a reliable sign that a letdown is occurring and milk flow has increased.