Progesterone

After ovulation the luteinised granulosa and theca cells of the corpus luteum secrete large amounts of progesterone (plus estradiol and inhibin A), peaking about a week after ovulation. If pregnancy occurs, hCG sustains the corpus luteum until the placenta takes over progesterone production around 7–9 weeks (the luteo-placental shift); thereafter placental progesterone rises steadily to term.

Progesterone acts on nuclear progesterone receptors (PR-A, PR-B), which must first be induced by estrogen. In the endometrium it stops proliferation and causes secretory transformation: coiled glands secreting glycogen-rich fluid, oedematous stroma, coiled spiral arteries and pre-decidual change — a receptive environment for implantation. It makes cervical mucus thick and impenetrable to sperm, relaxes smooth muscle (reduced myometrial contractility and oxytocin sensitivity — "progesterone block"; also slower gut and ureteric peristalsis in pregnancy), develops breast lobules and alveoli, raises basal body temperature by ≈0.3–0.5 °C (hypothalamic action) and stimulates ventilation (the mild respiratory alkalosis of pregnancy). It slows GnRH pulse frequency, reducing LH.

When the corpus luteum regresses without pregnancy, the fall in progesterone triggers prostaglandin release, constriction of spiral arteries and menstruation.