Progesterone is a hormone which plays a significant role in maintaining the early stages of pregnancy and helps in the development of the placenta. It is specifically responsible for the growth of blood vessels in the uterus lining which nourishes the embryo. Progesterone is also key during labor as it prepares the pelvic wall muscles. This is mainly secreted by the corpus luteum, the mass of cells forming in the ovary. Smaller quantities are also produced by the ovaries and the adrenal glands.