Progesterone is also secreted in lesser quantities by the adrenal gland and placenta. Under the influence of progesterone the uterus prepares for pregnancy. Progesterone is referred to as the hormone of pregnancy because it stimulates thickening of the lining of the uterus, stimulates development of the uterus prior to implantation, and because it is produced throughout pregnancy and maintains pregnancy. Blood levels of progesterone increase after ovulation and stay high until the end of diestrus. Levels of progesterone remain high if implantation occurs and pregnancy ensues, however, progesterone levels will decrease if pregnancy does not occur or just before parturition.
Progesterone also promotes complete development of the alveoli of the mammary gland.
Estrogens - These hormone are a group of compounds produced primarily by cells of the ovarian follicles, however they are also produced by the placenta and adrenal gland. They stimulate development and functioning of the female reproductive tract. Estradiol, one of the estrogens, is the hormone primarily responsible for the symptoms of estrus. The maximum concentration of estradiol is found around estrus. Estrogens also initiate growth and development of the mammary gland ducts.