The Science Behind Breastfeeding: How Milk Production Works

Breastfeeding is a pretty amazing phenomenon.  It has been sustaining the lives of humans and other mammals for thousands of years.  But do you know how breast milk production works?  With some help from our friends at, today we’re exploring the science behind breastfeeding – how you biologically produce that incredible breast milk that is so nourishing and beneficial to both your baby and yourself.

The process of lactation actually begins during pregnancy, as early as the second trimester in fact.  At this early stage, lactation is driven by hormones controlled by the endocrine system.  In the second trimester, lactogenisis begins, which is the initial internal production of colostrum.  Colostrum is the super-charged thick milky fluid first excreted by the mammary glands after birth.  But this colostrum remains in the body due to the high presence of progesterone during pregnancy.  After a baby is born, hormone levels, including progesterone and estrogen, drop significantly promoting the first secretion of colostrum.  Additionally, a hormone called prolactin encourages milk synthesis.  This second stage of lactogenisis usually begins within 2 days after giving birth, but breasts usually don’t feel full of milk until 2 to 3 days postpartum.

The Science Behind Breastfeeding:  How Milk Production WorksAfter the first few days, milk synthesis becomes less dependant on hormones and more controlled by the baby itself.  This is called autocrine control at the local level, that is, the breast.  Lactation is now based on the amount of milk a baby needs, and it’s all about supply and demand.  As the breast empties, it triggers production of more milk.  Usually a baby consumes about 80% of the breast milk a mother produces in a day, leaving 20% in her breast.  As milk is extracted, milk synthesis speeds up.  La Leche League likens this process to an automatic ice maker.  As the ice bin empties, the machine knows to make more ice to fill it back up to the top.  This process can continue indefinitely as long as milk removal continues to occur.

On a scientific level, milk cells called lactocytes are found on small cavities in the beast called alveoli.  Lactocytes carry prolactin receptors that encourage prolactin to flow through the bloodstream and stimulate the production of breast milk.  As milk fills the alveolus, the prolactin receptors are closed off due to expansion and signal a decrease in milk synthesis.  As the alveolus contract when milk is removed from the breast, prolactin receptors let the body know to produce milk again.  When breasts are filled and emptied frequently in the first few weeks after giving birth, prolactin receptors increase allowing the breast to produce and hold more milk throughout breastfeeding.

An understanding of milk synthesis is helpful when trying to increase milk supply.  Science tells us that emptying the breast more often throughout the day – whether that is through breastfeeding or pumping – will encourage more milk production.  Oxytocin is the hormone that controls let downs.  As the breast is stimulated, oxytocin typically kicks in within a minute to give a big release of breast milk.  Usually moms have more milk early in the day and less as the evening comes around.  However, we know that breast milk is changing constantly to meet the needs of the baby and this includes throughout the day.  Breast milk later in the day tends to have more fat, which helps babies sleep longer through the night and need less nighttime feedings.

When you’re cuddling your baby in your arms during feedings, breastfeeding may feel like a miracle. In truth, it is one of the miracles of human nature.  And while science is still trying to understand all of the elements involved, mothers and babies continue to benefit from this incredible biological phenomenon.