Progesterone levels are often used to determine if ovulation has occurred. The ovaries produce very small amounts of progesterone until ovulation happens. During the first half of the menstrual cycle (the follicular phase), progesterone levels are low – generally less than 3 ng/mL. Once ovulation occurs, the ovary starts producing progesterone in higher quantities and the blood progesterone level rises to above 3 ng/mL. To document ovulation, the progesterone level can be tested any time during the second half of the menstrual cycle (the luteal phase). Traditionally, progesterone is tested about a week after ovulation occurs. If your cycle is 28 days long, the progesterone level should be tested around day 21. If you have a 35-day cycle, the progesterone level should be tested on day 28.

The interpretation of the progesterone level is important. Blood progesterone levels vary greatly from hour to hour during the luteal phase and even at the peak (one week after ovulation). If you get your blood progesterone level tested a few times in the same day, the result will be different each time. Some physicians mistakenly believe that a progesterone level should be 10-12 ng/ml or higher to prove that a patient is ovulating or “ovulating well.” However, this is not true. Low progesterone levels (less than 3 ng/mL) can be seen any time before ovulation, immediately after ovulation, and just before menses begins, as serum progesterone is very pulsatile and can fluctuate 8-fold in as little time as 90 minutes.

The reference ranges and units used to measure progesterone levels can vary depending on the lab that analyzes the sample. However, for guidance, the reference ranges used if you measured your levels 7 days before your period might be:

• 30-65.5 nmol/L — this indicates that you’ve ovulated
• 15.2-30 nmol/L — this indicates that its possible youve ovulated but you might have done the test too early in your cycle
• 0-15.2 nmol/L — this indicates that you haven’t ovulated