“When a woman ovulates, but then does not become pregnant, falling progesterone levels trigger an inflammation response in the endometrium, and this inflammation response is not well understood, [Dr. Hilary] Critchley said. It is this inflammation that causes the breakdown and shedding of the endometrium. (The fluid that leaves a woman’s body may look like blood, but is actually a mix of fluid, red blood cells, immune system cells, the cells that line the uterus and cell fragments.)”

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Ray Peat (“Progesterone Summaries,” 2006-2016) wrote …

“It is important to emphasize that progesterone is not just the hormone of pregnancy. To use it only “to protect the uterus” would be like telling a man he doesn’t need testosterone if he doesn’t plan to father children, except that progesterone is of far greater and more basic physiological significance than testosterone.”