luteal phase

The luteal phase is the second half of the menstrual cycle, after the egg has been released. Its job is to make the body welcoming to a possible embryo. Where the follicular phase is about building and competing, the luteal phase is about maintaining a steady, nurturing environment and waiting to see whether pregnancy occurs.

After ovulation the emptied follicle reorganizes into the corpus luteum, a temporary gland that secretes large amounts of progesterone along with some estrogen. Progesterone converts the uterine lining into a secretory, nutrient-rich tissue ready for implantation, and its high levels feed back to suppress the pituitary, preventing new follicles from maturing. This is why a second ovulation does not normally occur in the same cycle.

If no pregnancy implants, the corpus luteum has a built-in lifespan of about 14 days; it then degenerates, progesterone and estrogen fall, and this hormonal withdrawal causes the lining to shed as menstruation, beginning a new cycle. A clinically relevant caveat is luteal phase deficiency, where inadequate progesterone or a short luteal phase can impair implantation and is sometimes treated with progesterone support in fertility care.