In some infants, the switchover is incomplete, and blood flow through the pulmonary arteries is inadequate. At the time of birth, and for a few days after, the mother's breasts contain a fluid called colostrum . It is rich in calories and protein, including antibodies that provide passive immunity for the newborn infant. Three or four days after delivery, the breasts begin to secrete milk. Its release is stimulated by a rise in the level of oxytocin when the baby begins nursing.
Milk contains an inhibitory peptide .
If the breasts are not fully emptied, the peptide accumulates and inhibits milk production. This autocrine action thus matches supply with demand.