"Day 21 progesterone" is the test almost everyone trying to conceive eventually orders. The question it answers is simple: did you actually ovulate this cycle? A value above 5 ng/mL says yes, the corpus luteum formed and is doing its job. Below that, you may have had a cycle that looked regular on paper and never actually released an egg. That is more common than people realize, and it is one of the louder explanations for "unexplained" infertility and the spotting that goes with it.
Outside that headline, progesterone matters mostly in pregnancy: rising sharply, supporting implantation, then handed off from the corpus luteum to the placenta around 8 to 10 weeks. The pregnancy reference ranges look nothing like the cycle ranges, which is why context drives interpretation. There are other roles (mood, sleep, body temperature, a precursor to cortisol) but in practice you draw progesterone for one of three reasons: confirming ovulation, evaluating early pregnancy, or chasing an abnormal cycle.
What progesterone measures
Progesterone is produced primarily by the corpus luteum after ovulation. The corpus luteum is what remains of the ruptured follicle, and its progesterone production is the dominant signal that the cycle has progressed past ovulation.
If pregnancy occurs, the corpus luteum continues progesterone production for about 8–10 weeks, after which the placenta takes over. If pregnancy does not occur, the corpus luteum involutes, progesterone falls, and menses follows.
Outside the reproductive cycle, progesterone is also synthesized in small amounts by the adrenal glands and serves as a precursor for cortisol and aldosterone synthesis.
Progesterone reference ranges
| Demographic | Low | High | Unit |
|---|---|---|---|
| Follicular phase | 0 | 1 | ng/mL |
| Mid-luteal (day 21, ovulatory) | 5 | 20 | ng/mL |
| Pregnancy, first trimester | 11 | 44 | ng/mL |
| Pregnancy, second trimester | 25 | 83 | ng/mL |
| Pregnancy, third trimester | 65 | 290 | ng/mL |
| Postmenopausal women | 0 | 0.5 | ng/mL |
| Adult men | 0 | 0.5 | ng/mL |
| Anovulation cutoff (luteal) | 0 | 3 | ng/mL |
Progesterone varies dramatically across the cycle and during pregnancy:
- Follicular phase (women): below 1 ng/mL.
- Ovulation (around day 14): rising, 1–3 ng/mL.
- Mid-luteal (day 21 of 28-day cycle): 5–20 ng/mL. Above 5 confirms ovulation; many endocrinologists prefer above 10.
- Pregnancy first trimester: 11–44 ng/mL.
- Pregnancy second trimester: 25–83 ng/mL.
- Pregnancy third trimester: 65–290 ng/mL.
- Postmenopausal women: below 0.5 ng/mL.
- Adult men: below 0.5 ng/mL.
For an irregular cycle, "day 21" should be timed instead as 7 days before expected next menses. A woman with a 35-day cycle should test on day 28, not day 21.
What high progesterone means
High progesterone almost always reflects ovulation, pregnancy, or hormone therapy. The differential:
- Pregnancy: the most common cause of progesterone above 20 ng/mL in a reproductive-age woman.
- Normal luteal phase: values 5–25 ng/mL in the week after ovulation.
- Multiple pregnancy: twin or higher-order gestations have higher progesterone.
- Ovarian cyst (luteal cyst): persistent corpus luteum producing progesterone.
- Progesterone supplementation: common in IVF cycles, luteal support, hormone therapy.
- Congenital adrenal hyperplasia: particularly 21-hydroxylase deficiency, with elevated 17-OH progesterone (a related steroid).
- Progesterone-secreting ovarian tumor: rare.
In a postmenopausal woman not on hormone therapy, progesterone above 1–2 ng/mL warrants explanation.
What low progesterone means
Low progesterone in the luteal phase or early pregnancy is the clinically important pattern.
- Anovulation: no ovulation, no corpus luteum, no progesterone surge. The most common explanation for day 21 progesterone below 5 ng/mL with apparent regular cycles.
- Luteal phase deficiency: ovulation occurs but corpus luteum function is inadequate. Real but over-diagnosed; modern reproductive endocrinology is skeptical of "LPD" as a routine cause of infertility.
- Hypothalamic amenorrhea: no cycling, no progesterone.
- Polycystic ovary syndrome: frequent anovulation; many cycles never produce a luteal phase.
- Hyperprolactinemia: disrupts ovulation; low prolactin-driven progesterone via anovulation.
- Premature ovarian insufficiency: failing ovaries, low progesterone with high FSH.
- Threatened miscarriage / ectopic pregnancy: abnormally low progesterone (<5 ng/mL) in early pregnancy is associated with worse outcomes; below 5 ng/mL with abdominal pain raises concern for ectopic.
- Wrong test timing: by far the most common reason. A "low" progesterone drawn on day 7 just means the woman was in the follicular phase.
Note on ovulation kits and progesterone metabolites: home urine PdG (pregnanediol glucuronide) tests offer a reasonable confirmation of ovulation without lab draws.
Day-21 testing, getting it right
"Day 21 progesterone" is shorthand for "draw progesterone roughly 7 days after ovulation", which assumes a 28-day cycle and ovulation on day 14. That assumption is often wrong. Better practice:
- For a regular 28-day cycle: day 21 is fine.
- For longer cycles: count 7 days back from expected menses. A 35-day cycle = test on day 28.
- For irregular cycles: multiple draws (day 21, 25, 28) or use ovulation predictor kits to time the draw 7 days post-LH surge.
- Above 5 ng/mL = ovulation occurred. Above 10 ng/mL = solid luteal phase. Below 3 ng/mL = anovulatory.
Single-point progesterone has limitations. Pulsatile secretion produces fluctuations of several ng/mL across hours. A single value just under threshold should be confirmed on a repeat cycle before drawing conclusions.
Track this biomarker over time in AskAnything.health — upload your lab results and see trends at a glance.
When to act on progesterone
- Day 21 progesterone below 5 ng/mL in a woman trying to conceive, confirm with timing review and a repeat cycle; investigate anovulation.
- Early pregnancy progesterone below 5 ng/mL: concerning for nonviable pregnancy or ectopic; correlate with hCG trend and ultrasound.
- High progesterone in postmenopausal woman not on HT: investigate luteal cyst or rare tumor.
- Recurrent miscarriage with low luteal progesterone: luteal support is often given empirically, though high-quality evidence of benefit is limited outside IVF cycles.
- Persistent anovulation: workup for PCOS, hypothalamic amenorrhea, hyperprolactinemia, thyroid disease.
This information is for educational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider about your lab results.
Tests that complete the picture
- Estradiol: paired with progesterone in luteal phase; estradiol drives endometrial proliferation, progesterone stabilizes it.
- LH: ovulation surge timing; high in PCOS.
- FSH: ovarian reserve and POI.
- Prolactin: common cause of anovulation.
- TSH: thyroid disease causes irregular cycles.
- AMH: ovarian reserve.
- 17-OH progesterone: distinct test for congenital adrenal hyperplasia.
- hCG: confirms or excludes pregnancy when progesterone is unexpectedly elevated.