Progesterone is a hormone that rises after ovulation to help prepare the uterine lining for pregnancy.
What progesterone does
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 so it is ready for a pregnancy. If no pregnancy occurs, progesterone falls, and this fall is what triggers a period. Because of this, the presence of progesterone becomes a marker that ovulation has taken place. Progesterone also works with estrogen to keep the cycle regular, and the balance between these two hormones matters for healthy periods. The hormonal background is explained further on the hormones page.
Why it matters in PCOS
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 and can keep thickening. This can lead to infrequent periods, and sometimes a heavier bleed when one finally comes. A lining that thickens for too long is also one reason doctors pay attention to very infrequent periods. This pattern varies from person to person, so one person’s experience may differ from another’s. The link between ovulation and irregular periods is explained on the what is PCOS page.
How progesterone is assessed
A progesterone blood test timed within the cycle can help a doctor judge whether ovulation happened. The timing has to fit each person’s cycle, so interpretation depends on context. When cycles are irregular, choosing the right time to test becomes harder, because the day of ovulation cannot be predicted easily. A single result does not capture the whole picture, and it is read together with symptoms and other test results. A doctor may need to repeat the test at a different time for a clearer view.
When to talk to a doctor
If your periods are infrequent or you are planning a pregnancy, a doctor can explain whether progesterone assessment suits you. Any decision about hormone medicines should be discussed with a doctor or pharmacist, since it depends on your health and your goals. More about fertility is on the PCOS and fertility page.
This glossary is education, not diagnosis. For your own situation, ask a doctor or pharmacist.