Day 21 Progesterone When Trying to Conceive
A mid-luteal (day 21) serum progesterone level of at least 10 ng/mL (30 nmol/L) confirms ovulation and adequate luteal function when trying to conceive naturally, with optimal fertility occurring in the range of 10-16 ng/mL.
Optimal Progesterone Range for Natural Conception
The lower threshold of 10 ng/mL (30 nmol/L) represents the minimum level needed to confirm ovulation and potential fertility in a natural conception cycle, based on landmark data from untreated conception cycles 1.
In 21 untreated singleton conception cycles, the mean progesterone was 12.8 ng/mL with a range of 8.5-16.7 ng/mL, demonstrating that there is both an upper and lower limit to the optimal fertility range 1.
The lower limit should be taken as at least 9.4 ng/mL (30 nmol/L) to allow for assay variation and provide a clinically reliable criterion of ovulation 1.
Clinical Interpretation by Progesterone Level
Progesterone <10 ng/mL: Suggests inadequate luteal function or anovulation, associated with significantly reduced conception rates 1, 2.
Progesterone 10-16 ng/mL: Represents the optimal range observed in natural conception cycles, with the highest likelihood of successful pregnancy 1.
Progesterone >20 ng/mL: While indicating robust ovulation, levels above 20-25 ng/mL are more predictive of viable pregnancy once conception has occurred rather than being necessary for initial conception 3.
Important Caveats
Timing is critical: The progesterone must be measured at the true mid-luteal phase (approximately 7 days after ovulation, or day 21 of a 28-day cycle) 1.
For women with irregular cycles, timing the test based on ovulation predictor kits or ultrasound monitoring is more accurate than assuming day 21 1.
A single low value does not definitively diagnose luteal phase defect; repeat testing in subsequent cycles may be warranted before concluding there is a persistent problem 1.
The progesterone threshold may be higher after ovulation induction with clomiphene or gonadotropins due to contributions from multiple stimulated follicles 1.