While a woman is pregnant, she has a high level of hormones Oestrogen and Progesterone levels in the body. Once the placenta (afterbirth) is expelled, these hormone levels decrease rapidly. This allows the hormone Prolactin, which has been rising in the pregnancy, to be released in greater quantity. Prolactin stimulates the body to make milk. The milk is produced in specially designed cells called Alveoli in the breasts. The milk is transported to the nipple along the breast ducts.
Although this is a natural process, certain factors can affect when and how much milk is produced, such as a woman’s thoughts on breastfeeding and if there is any stress of anxiety. However, most women will find that producing milk will start to happen from about 2-6 days after the birth.
Ref:'Supply' and 'demand': breastfeeding as labour - Social Science and Medicine, vol 60, no 10, May 2005, pp 2283-2293 Dykes F - (2005)