Lactation is the process of milk production and secretion from the mammary glands, which are exocrine glands located in the breasts. In female adult humans, milk production begins during pregnancy, and lactation continues throughout the postpartum period. Initially, hormones (like progesterone and prolactin) regulate milk production. Milk removal takes on a greater role in stimulating ongoing lactation by four days postpartum, and by ten days postpartum, ongoing lactation is entirely stimulated by removal of milk from the breasts. Lactation insufficiency occurs when the parent does not produce enough milk to meet the nutritional needs of their infant.