The way a newborn baby 'milk's the breast' is an instinctive skill they are born with (although attaching your baby to the breast correctly, can sometimes be a bit tricky!) There are 2 mechanisms that need to come into play for a baby to be able to withdraw all the milk they need from their mother's breast. These are:
The 'let-down' reflex
Suckling at the breast
The 'let-down' reflex.
The 'let-down' reflex is a response by the woman's body to the baby suckling. The stimulation of her nipple (and the woman's relaxation) triggers the release of the natural oxytocin hormone from her brain, causing her milk to 'eject', or flow freely. The 'let down' is also known as the 'milk ejection reflex', and usually starts around the time that the milk 'comes in' about 2 to 5 days after the birth.
Oxytocin is the same hormone that stimulates the contractions of the woman's uterus during labour (or the synthetic form, Syntocinon used to induce labour). Therefore, when a let-down occurs, the uterus also naturally contracts. This can cause slightly heavier, or brighter, bleeding after a feed in the first few weeks after the birth, and for some women it can cause strong afterpains for a few days (usually being more common with second or subsequent babies).
The let-down is usually triggered about 1 to 2 minutes after the baby starts feeding (or the woman starts expressing her milk). When the oxytocin hormone is released, it causes small muscle fibres around the breast's full milk ducts to contract for up to a minute or so, forcefully pushing the pooled milk down and out the nipple, often quite rapidly. A let-down can also happen if the woman is looking at her baby, thinking about her baby, if she hears her baby (or another baby) cry, or her nipples rub up against something. When the woman resumes sex after the birth, a let-down can also occur during touching of the nipples and the breasts and/or if she experiences an orgasm (take a towel with you!).