Colostrum (also known as first milk) is the milky fluid secreted by the mammary glands shortly before and after delivery. This is the first substance produced by the mammary gland and appears for two or three days after delivery. This watery fluid contains proteins and antibodies. As colostrum is rich in antibodies, it protects the newborn against disease until the baby's own immune system is matured.
A hormone oxytocin* which targets uterine muscles and mammary glands is responsible for the contractions and stimulates flow of milk from breasts during lactation.
*The hormone oxytocin is made by nerve cells in the hypothalamus.