How Breastfeeding Works: How Your Body Makes and Releases Milk

Breastfeeding looks simple from the outside, but a lot happens inside your body to turn a baby’s sucking into a steady supply of milk. Knowing the basics makes everything else easier to follow, from why newborns feed so often to why a missed feed can leave you uncomfortable.

This guide covers the parts of the breast, how milk is made from pregnancy onward, what the two main hormones do, and why supply follows demand. If you’re looking for hands-on help, start with our guide to how to breastfeed and the wider breastfeeding basics hub.

What’s inside the breast

Cleveland Clinic describes the milk-making system as a tree. The leaves are the alveoli, tiny grape-like sacs that make and store milk. Clusters of alveoli form lobules, lobules join into lobes, and milk ducts carry milk from each lobe toward the nipple, which has small pores where milk comes out. The darker skin around the nipple (the areola) is rich in nerve endings that sense suckling.

How many ducts there are is less clear than older textbooks suggest. Cleveland Clinic says you can have up to 20 lobes with one duct each. An ultrasound study of 21 breastfeeding women found an average of about 9 to 10 main ducts per breast, about 2 mm (0.08 in) wide, that branch close to the nipple. The researchers concluded that ducts mainly transport milk rather than store it, and that the mix of glandular and fatty tissue varied widely between women without being linked to how much milk they made.

How milk is made, stage by stage

Researchers describe lactation in stages. The timing below combines what Cleveland Clinic, the NHS, the AAP and two physiology reviews say.

Stage	When	What happens
Building the system	Pregnancy (Cleveland Clinic puts the start around week 16)	Estrogen and progesterone grow the milk ducts. Your breasts begin making colostrum.
Colostrum	The first few days after birth	Small amounts of thick, concentrated first milk.
Milk “comes in”	About day 2 to 5	Breasts feel fuller as milk volume rises and colostrum gives way to transitional, then mature milk.
Established lactation	From a few weeks on	Production is driven mainly by how much milk is removed.

Colostrum. The NHS says your baby needs only about a teaspoon at each feed in the first days, and the AAP notes that you already have enough colostrum to nourish your baby on the day of birth. The ABM lists average reported intakes by healthy term babies: 2 to 10 ml per feed in the first 24 hours (about ½ to 2 teaspoons, our conversion), rising to 5 to 15 ml in hours 24 to 48, 15 to 30 ml (up to 1 oz) in hours 48 to 72 and 30 to 60 ml (1 to 2 oz) in hours 72 to 96. These are averages, and the ABM says feedings should follow your baby’s cues. Our colostrum guide goes deeper.

Milk coming in. Neville and Morton describe “lactogenesis II”, the onset of copious milk secretion, as taking place during the first 4 days after birth. After the placenta is delivered, progesterone drops, and that withdrawal is the trigger, as long as prolactin and adequate cortisol are present. The NHS says breasts typically feel fuller around 2 to 4 days after birth, and the AAP says colostrum gives way to a larger volume of transitional and then mature milk about 2 to 5 days after birth. The review also notes that this timing can be delayed after stressful deliveries and with poorly controlled diabetes. See when does breast milk come in for what to expect and when to ask for help.

Neville and Morton also report that failure to remove colostrum from the breast early is associated with poorer prospects for successful lactation in many women. The mechanism isn’t proven, but it fits with the standard advice to feed often from the start.

Two hormones do most of the work

• Prolactin tells the alveoli to make milk. Cleveland Clinic explains that pregnancy hormones hold it back until the placenta is delivered, after which prolactin takes charge.
• Oxytocin releases the milk. When your baby suckles, nerves in the nipple signal your brain, and Cleveland Clinic says that triggers both hormones. Oxytocin causes muscle contractions that push milk out of the alveoli and through the ducts. That release is the let-down reflex.

The NHS adds that it matters to breastfeed at night in the early weeks, because prolactin, which builds your milk supply, is produced then.

Why supply follows demand

Once breastfeeding is established, your body mostly responds to how much milk leaves the breast, not to how much your baby could theoretically take. The NHS puts it this way: each time your baby feeds, your body knows to make more milk for the next feed, and the amount rises or falls with how often your baby feeds.

Researchers Daly and Hartmann reviewed studies of breastfeeding and expressing mothers and concluded that, with demand feeding, milk production is likely to reflect the baby’s appetite rather than the mother’s ability to produce milk, which may be several times higher. Their model suggests the breast responds to how fully the baby empties it at each feed.

Real life is variable, and that’s normal. In a study of 71 mothers of babies aged 1 to 6 months, babies fed an average of 11 times in 24 hours, but the range ran from 6 to 18 feeds. An average feed was about 2.7 oz (76 g), and the left and right breasts rarely produced the same amount. Night feeds made an important contribution to total intake. There was no relationship between the number of feeds per day and the mothers’ 24-hour milk production, and the authors concluded that babies should feed on demand, day and night, rather than conform to an average.

What this means in practice

• Feed on cue, day and night. The NHS suggests feeding as often and as long as your baby wants, and says you can’t overfeed a breastfed baby.
• Remove milk when baby can’t. If you’re apart or your baby can’t feed well, expressing signals your body the same way. See our pumping schedule guide.
• Be careful with early top-ups. The NHS says that in the early weeks formula top-ups or a pacifier can lower your supply. If a doctor has advised supplementing, follow that advice and ask for help protecting your supply.
• Get help early if you’re worried. The NHS advises speaking to a midwife or health visitor if you think your baby isn’t getting enough milk. Our guides to how to increase milk supply and how often to breastfeed explain what to look for.
• Expect cluster feeding. Bunches of close-together feeds are common, and cluster feeding is often a normal part of the pattern.

When to call a doctor or lactation consultant

Ask your midwife, pediatrician or a lactation consultant (IBCLC) for help if:

• You’re worried your milk hasn’t come in
• Your baby has few wet or dirty diapers, seems very sleepy or isn’t gaining weight
• Feeding is painful or your baby keeps slipping off the breast
• You have a fever, flu-like symptoms or a hot, red, painful area on a breast

Our breastfeeding support page lists IBCLCs, La Leche League, WIC and helplines. For more on making milk, visit the milk supply hub.