A particular hormone responsible for stimulating alveoli increases in concentration in the last trimester of pregnancy. The hormone is

A principal hormone in promoting milk production is prolactin (PRL), which is secreted from the anterior pituitary gland. Even though prolactin levels increase as the pregnancy progresses, no milk production occurs because progesterone inhibits the effects of prolactin. After delivery, the levels of estrogens and progesterone in
the mother’s blood decrease, and the inhibition is removed. Prolactin promotes the growth of mammary alveoli, which are the components of the mammary gland, where the actual production of milk occurs. Prolactin stimulates the breast alveolar epithelial cells to synthesize milk components, including lactose (i.e., the carbohydrate of milk), casein (i.e., the protein of milk), and lipids.