Breast milk is made in small sacs called alveoli and flows through ducts to the nipple. Supply works on demand: the more milk is removed, the more is made.
Breast structure
Each breast has 15 to 20 lobes of milk-making alveoli that drain through ducts to the nipple. Fat and connective tissue set the size of the breast, which does not predict milk supply.
Prolactin makes milk
Suckling signals the pituitary to release prolactin, which makes the alveoli produce milk. Frequent feeding, 8 to 12 times a day, keeps the supply up.
Oxytocin lets milk flow
Oxytocin squeezes the cells around the alveoli and pushes milk out, the let-down reflex. It also contracts the uterus, which helps limit bleeding after birth.
Colostrum and mature milk
The first milk, colostrum, is small in volume and rich in antibodies. Milk volume increases over about 2 to 5 days after birth.
At the bedside
A good latch takes in much of the areola, not just the nipple, with the baby's lips flanged out; nipple pain often means a shallow latch. Cramping while nursing is oxytocin contracting the uterus.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).