Progesterone is a steroid hormone produced primarily by the corpus luteum — the temporary endocrine structure that forms in the ovary after ovulation — and, during pregnancy, by the placenta. Its primary roles are to prepare and maintain the uterine lining for embryo implantation, suppress further ovulation during pregnancy, and support early pregnancy development.

In clinical practice, a mid-luteal progesterone test (taken around day 21 of a 28-day cycle, or 7 days before the expected period) is used to confirm that ovulation has occurred. Low progesterone in the luteal phase is associated with irregular cycles, infertility, and early pregnancy loss. Progesterone is also used in both men and women to balance the effects of oestrogen in hormone replacement therapy.