Why the milk waits until birth
Throughout pregnancy, prolactin from the anterior pituitary climbs steadily, and the breast is fully built and ready. Yet little milk is made. The reason is a brake: the high progesterone of pregnancy blocks prolactin's milk-making action. The breast is loaded but the safety is on.
Delivery of the placenta removes its hormones almost overnight. Progesterone and estrogen plunge, the brake is released, and prolactin — still high — finally switches on full milk production. Milk usually 'comes in' a few days after birth. Before then the breast makes colostrum, a small-volume, antibody-rich first food.
Two hormones, two different jobs
It is worth separating the two hormones of breastfeeding because they are easy to confuse. Prolactin makes milk: it tells the gland cells to synthesize the milk that fills the breast between feeds. Oxytocin ejects milk: it squeezes the tiny muscle cells around the milk sacs so the stored milk is pushed out toward the nipple. One fills the tank; the other opens the tap.
The oxytocin step is the let-down reflex. Suckling at the nipple sends nerve signals to the brain, the posterior pituitary releases a pulse of oxytocin, and within seconds milk lets down. Because it is wired through the brain, let-down can be triggered just by hearing the baby cry or thinking about feeding — and it can be blocked by stress or pain, which is why a calm, comfortable feed matters.
Supply follows demand
- The baby suckles, stretching sensory nerves in the nipple and areola.
- Those nerves signal the hypothalamus, which raises prolactin (more milk made for next time) and triggers oxytocin release (milk let down now).
- More suckling → more prolactin → more milk. Less suckling → less prolactin → supply fades.
This demand-driven loop is why frequent feeding builds supply and why milk gradually stops once feeding ends. The same persistently high prolactin that maintains milk also suppresses GnRH, which dampens LH and FSH and so pauses ovulation. That natural pause is lactational amenorrhea — a partial, unreliable spacing of pregnancies built into breastfeeding itself.