Colostrum
Breast milk
The main hormone responsible for the production of breast milk is called 'prolactin'. Prolactin is produced by the woman's anterior pituitary gland in her brain. By the middle of pregnancy, prolactin levels are up to 20 times higher than they were before conception. However, despite these very high levels, the milk-producing cells in the woman's breasts do not respond (by producing mature milk) because of the high levels of progesterone hormone being produced by the
placenta.
After the baby's birth, the placenta is delivered and the levels of progesterone decrease fairly rapidly over the following 48 to 72 hours (yet remain higher than they were before the pregnancy). About 2 to 5 days after the birth, the progesterone drops to a level where it can no longer inhibit the prolactin hormone. The high levels of prolactin are then able to stimulate the milk producing cells in the breasts to produce mature milk, being seen as the woman's milk 'coming in'.
Colostrum
Colostrum is the first fluid a woman's breasts produce and is present from about 16 to 24 weeks of the pregnancy, depending on the woman's individual body. Colostrum is a clear or creamy-yellow substance that is syrupy in consistency and is specially designed as the baby's first food once they are born.
Babies thrive on colostrum until their mother's mature milk comes in (sometime between days 2 to 5 after the birth). Colostrum contains more proteins, minerals and
fat-soluble vitamins than mature milk (but less lactose, fats and
water-soluble vitamins). It also contains more anti-infective agents (or 'antibodies'), that help protect babies until their own immune systems mature. Colostrum is lower in volume than mature milk, so it does not overfill your baby's very small stomach (about the size of a golf ball at birth!)
It is possible for some women to hand express colostrum from their nipples during the pregnancy, or to occasionally leak colostrum.