Progesterone
Marker of ovulation and luteal phase adequacy
What it is
Progesterone is a hormone produced primarily by the corpus luteum — the structure that forms in the ovary after a follicle releases an egg. Its main function is to prepare the endometrium (uterine lining) for embryo implantation and to support an early pregnancy.
In the absence of ovulation, the corpus luteum does not form and progesterone remains very low throughout the cycle. This is precisely what happens in most anovulatory cycles in PCOS.
Why it matters in PCOS
A progesterone level drawn on day 21 of the cycle (for a 28-day cycle) is the simplest way to confirm that ovulation has occurred. A level of ≥ 16 nmol/L (approximately 5 ng/mL) in the mid-luteal phase is considered probable evidence of ovulation. Below this threshold — particularly below 10 nmol/L — ovulation is doubtful or absent.
In PCOS, cycles are often anovulatory: progesterone remains low throughout, the endometrium is not properly transformed, and periods are absent or erratic. Some people with PCOS ovulate occasionally but may have a short or insufficient luteal phase (luteal phase deficiency), which progesterone testing can help identify.
The progesterone challenge test is a diagnostic tool sometimes used in amenorrhoea: a progestogen is given for 10 days; if a withdrawal bleed follows, this confirms that the endometrium is responsive (adequately oestrogenised) and that the amenorrhoea is due to anovulation, as in PCOS.
Normal values
- Follicular phase: < 3 nmol/L (< 1 ng/mL)
- Day 21 (mid-luteal) — probable ovulation: ≥ 16 nmol/L (≥ 5 ng/mL)
- Full luteal phase: 16–95 nmol/L
- First trimester pregnancy: 35–145 nmol/L
For irregular cycles, day-21 testing may be poorly timed. It is better to draw 7 days after presumed ovulation (detected by basal body temperature charting or LH testing).
When is it measured?
In practice, progesterone is measured on day 21 of a 28-day cycle, or 7 days after presumed ovulation if the cycle is irregular. In IVF or ovulation induction protocols, it is measured at set timepoints to guide luteal support (progesterone supplementation).
Key takeaways
- Produced by the corpus luteum after ovulation
- Day-21 ≥ 16 nmol/L = probable ovulation; < 10 nmol/L = ovulation doubtful
- In PCOS: often low due to frequent anovulatory cycles
- Progesterone challenge test confirms anovulatory amenorrhoea
- Adjust timing of blood draw to actual cycle length