Ideal CD21 Progesterone for Conception
For women trying to conceive naturally, a mid-luteal (cycle day 21) progesterone level of at least 10 ng/mL (30 nmol/L) indicates ovulation has occurred, though optimal fertility appears to require levels between 10-16 ng/mL (30-50 nmol/L). 1, 2
Evidence-Based Progesterone Thresholds
Minimum Threshold for Ovulation
- A progesterone level below 6 nmol/L (<2 ng/mL) indicates anovulation, which is a common cause of infertility and requires investigation for conditions like polycystic ovary syndrome (PCOS) or hypothalamic amenorrhea 1
- The guideline recommendation uses <6 nmol/L as the cutoff for anovulation when evaluating infertility 1
Optimal Range for Conception
- The mean progesterone in untreated conception cycles was 12.8 ng/mL (40.7 nmol/L), with a 95% confidence interval of 8.8-16.7 ng/mL (28-53 nmol/L) 2
- This research demonstrates both an upper and lower limit exist for optimal fertility, not just a minimum threshold 2
- A practical lower limit of 9.4 ng/mL (30 nmol/L) is recommended to account for assay variation and provide a clinically reliable criterion of potential fertility 2
Clinical Interpretation Algorithm
For CD21 progesterone results:
<6 nmol/L (<2 ng/mL): Anovulation confirmed - investigate underlying causes (PCOS, hypothalamic dysfunction, hyperprolactinemia, thyroid disease) 1
6-10 ng/mL (20-30 nmol/L): Suboptimal luteal function - may indicate inadequate corpus luteum function requiring further evaluation 1, 2
10-16 ng/mL (30-50 nmol/L): Optimal range for conception - confirms ovulation with adequate luteal support 2
>16 ng/mL (>50 nmol/L): Above typical conception range - while ovulation occurred, excessively high levels may not be optimal 2
Important Caveats
Timing Considerations
- Blood must be drawn during the mid-luteal phase (approximately 7 days after ovulation, which is cycle day 21 in a 28-day cycle) 1
- For irregular cycles, timing should be adjusted based on ovulation prediction rather than assuming day 21 1
- Multiple measurements over 6 months provide better assessment than a single value for women with menstrual irregularity 1
When Higher Levels May Be Required
- After clomiphene or gonadotropin treatment, higher progesterone levels may be needed due to contributions from multiple stimulated follicles 2
- This does not apply to natural conception cycles 2
What NOT to Do
- Do not routinely supplement progesterone based solely on serum levels in early pregnancy - current guidelines do not recommend progesterone testing or treatment decisions based on hormone values alone in established pregnancies 3
- Progesterone supplementation is indicated for specific clinical scenarios (prior preterm birth, short cervix), not for low progesterone levels per se 3
When to Investigate Further
If progesterone is low (<10 ng/mL), evaluate for: