After a mother delivers her baby and the placenta, the level of progesterone
in the body drops drastically, stimulating the relase of prolactin in the body. Prolactin is the hormone that stimulates the
breasts to make milk. Specifically, prolactin stimulates the milk glands (alveoli) to make the milk. When the milk is released,
it flows through ducts and sinuses (small milk storage spaces) in the breast and out through the nipples.
As the nipples are stimulated (by the infant sucking, or a breast pump),
another hormone, oxytocin, is released. This hormone causes the sinuses to constrict (specifically, the myoepithelial cells
around them, pictured above, contract around the sinuses), forcing the milk forward toward the nipples. This is called the
'let-down reflex', and makes the milk available for the baby. The let-down reflex is generally triggered by nipple stimulation,
but amazingly, it can also be triggered simply by thinking about the baby, or even a situation such as hearing a baby cry
at the grocery store.
The small amount of milk available before the let-down reflex occurs
at each feeding is called foremilk. Hind milk, the milk that is stimulated by the baby's sucking (released after the let-down
reflex), is the milk that is higher in fat and causes the most infant growth.
Breastfeeding is very much a 'supply and demand' process. The breasts
will supply adequate milk, but the demand has to be present for the milk. So if a baby is not eating very much,
the mother's body will sense this and decrease its milk supply. Thus, if a baby is sick or unable to breastfeed for a
period of time (or if the mother is bottle feeding sometimes) for some other reason, it is important for a mother to use a
breast pump to continue that 'demand' from the breasts, if she wishes to resume breastfeeding full time.
It is also important to note that, if a mother wishes to breastfeed her
baby for the majority of feedings, bottles and pacifiers shoud be avoided for the first four weeks of breastfeeding. The baby
may grow used to the shape of the bottle/pacifier nipple and not breastfeed as well if bottles and pacifiers are started too
Mothers will notice that their breast milk changes in the days after
their baby's birth. A thin, watery type of milk, colostrum may begin to leak from the breasts as soon as the fourth month
of pregnancy. Colostrum provides total nutrition for the baby, and because babies are born with very small stomachs and generally
an extra fluid volume, the low volume of colostum the baby receives is usually enough. This fluid, yellowish in color, begins
to change to transitional milk in the third or fourth day after birth, and becomes mature or true breastmilk by the tenth
day after birth. This milk is 90% water and 10% solid, and higher in fat and vitamins than colostrum, providing the growing
baby with the nutrients he needs to develop and mature.