Progesterone (P4)
The hormone of the second half of the cycle and of pregnancy. A mid-luteal level is the classic proof that ovulation happened. It prepares and maintains the uterine lining and is the most direct blood confirmation of an ovulatory cycle.
What it measures
Progesterone is produced mainly by the corpus luteum — the structure left behind after an ovarian follicle releases its egg — and later, in pregnancy, by the placenta. Before ovulation it is very low; after ovulation it rises sharply for about a week, then falls if there is no pregnancy, triggering menstruation. A blood level drawn around day 21 of a 28-day cycle (about 7 days before the next period) is the standard test to confirm ovulation. In pregnancy it climbs into the hundreds and sustains the lining. It is measured to investigate infertility, confirm ovulation, evaluate recurrent miscarriage, and assess corpus-luteum function.
What a high value can mean
- Ovulation has occurred — a mid-luteal value >3 ng/mL confirms it; this is the intended reading.
- Pregnancy — rises progressively.
- Congenital adrenal hyperplasia — an enzyme block shunts steroids toward progesterone precursors.
- Ovarian cysts (corpus-luteum cyst) — can prolong elevation.
- Progesterone supplementation — exogenous, expected.
What a low value can mean
- Anovulation — no ovulation, so no corpus luteum; the commonest cause of a low day-21 value (seen in PCOS, thyroid disease, hyperprolactinaemia, hypothalamic amenorrhoea).
- Luteal phase deficiency — corpus luteum forms but underperforms.
- Menopause / ovarian insufficiency.
- Wrong test timing — a level drawn too early or in an irregular cycle looks falsely low; the day-21 assumption fails when cycles aren't 28 days.
When to discuss with a doctor
The single most useful use of progesterone is a mid-luteal draw to confirm ovulation — timed 7 days before the expected next period, not blindly on day 21, because that assumption breaks with irregular cycles. A value >3 ng/mL means you ovulated; a low value in a woman trying to conceive prompts the anovulation work-up (thyroid, prolactin, androgens, PCOS assessment). Very high progesterone in a non-pregnant woman with virilisation should prompt evaluation for congenital adrenal hyperplasia. Mediora.AI shows progesterone alongside its cycle timing and companion hormones; the fertility interpretation belongs with your gynaecologist or fertility specialist.