Throughout pregnancy a woman's body produces hormones which stimulate the growth of the milk duct system in the breasts: progesterone, estrogen, follicle stimulating hormone (FSH), luteinizing hormone (LH), prolactin, oxytocin, and human placental lactogen (HPL). By the fifth or sixth month of pregnancy, the breasts are sufficiently developed to produce milk. Near the time of birth, the breasts may begin to secrete a thick, yellowish fluid called colostrum (or "beestings"), which is the first milk the infant receives. It contains important antibodies from the mother's body providing immunological protection. Colostrum has no fat and little sugar - these substances appear three to four days after birth when the suckling action of the infant further stimulates the breast to produce mature breast milk. Thereafter the breast produces milk on a basis of supply and demand in response to how often a child feeds and how much milk he or she consumes. The production, secretion and ejection of milk is called lactation.
The exact integrated properties of breast milk are unknown, but the nutrient content after this period is relatively consistent and draws its ingredients from the mother's food supply. If that supply is found lacking, content is obtained from the mother's bodily stores. The exact composition of breast milk varies from day to day, depending on food consumption, meaning that the ratio of water to fat fluctuates. Foremilk, the milk released at the beginning of a feeding, is low in fat and high in carbohydrates relative to the hindmilk which is released as the milk supplies are drawn down. The breast can never be truly "emptied" since milk production is a continuous biologic process.
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