A practical 2026 guide to ovulation, hormone testing, fertility, and the conditions that can lower progesterone Medically reviewed sources consulted: National Library of Medicine, American Society for Reproductive Medicine, and American College of Obstetricians and Gynecologists. The most useful way to understand what causes low progesterone is to follow the hormone back to its source. In a typical menstrual cycle, progesterone stays low before ovulation, rises after an egg is released, and falls again if…