Serum progesterone measurements are a reliable way to document ovulation. The human body produces very little progesterone, except during the luteal phase (two week period between ovulation and menses).
Ideally, progesterone is measured at its peak – usually seven days prior to expected menses. Thus, in a 28-day cycle, we would measure on day 21. In a 35-day cycle, we would measure on day 28.
Progesterone level interpretation
In our experience, progesterone level interpretation is one of the most misunderstood fertility evaluations, by patients and doctors alike. A common misconception is that a progesterone must be 10 ng/ml or greater to confirm good quality ovulation. In truth, there is a lot of variation in progesterone levels over the course of 90 minutes. Some studies have shown that progesterone levels can vary from as low as 2 ng/ml to 40 ng/ml within the same day.
A low progesterone level could mean that the level was drawn close to ovulation or close to menses (rather than at the peak in the mid-luteal phase). Additionally, a low progesterone level could simply mean that the blood was drawn at a time when the level briefly fluctuated down.
Since it is not feasible to continuously sample a woman’s blood and plot the change in her progesterone levels throughout her luteal phase, progesterone levels are best used to confirm that ovulation occurred (when levels are > 3 ng/ml).
Beyond that, the length of the luteal phase is more informative about adequacy of progesterone production than the absolute progesterone level.
Progesterone levels and miscarriage
Patients suffering from prior miscarriage(s) commonly ask whether they need supplemental progesterone in pregnancy, because they were told during prior miscarriages that their progesterone level was “low.” In fact, low progesterone levels in pregnancy are almost always a symptom of an abnormally developing pregnancy (either in the uterus or in the tubes) rather than a cause of miscarriage.