In simple terms: a hormone that rises after ovulation and helps prepare the uterine lining.
Why it matters for PCOS
Progesterone is made by the corpus luteum, the structure that forms on the ovary after an egg is released at ovulation. In the second half of the cycle, it thickens and stabilises the uterine lining. If no pregnancy occurs, progesterone falls and triggers a period, which makes it a marker that ovulation has taken place.
In PCOS, ovulation is often irregular or absent, so progesterone may be low or inconsistent. Without a regular progesterone rise, the uterine lining does not shed on schedule, leading to infrequent periods and sometimes a heavy bleed when one finally comes. Doctors sometimes prescribe progesterone to bring on a period, and a progesterone blood test around day 21 can help confirm whether ovulation happened.
This glossary is education, not diagnosis. For your own situation, ask a doctor or pharmacist.