On Day 21 of your cycle, your fertility specialist may want to check the levels of progesterone and estradiol (E2) in your system and the thickness of your endometrium (uterine lining).
Day 21 testing checks a woman’s progesterone level to confirm that ovulation has occurred. It is done on the 21st day of the menstrual cycle (Day 1 is the first day of flow, not including any spotting). A low Day 21 progesterone level suggests the cycle was anovulatory (no egg was produced). If no egg is produced, pregnancy cannot be achieved.
The timing of ovulation, and the associated peak in progesterone, is related to the subsequent menstrual period, not the preceding one. In an average cycle of 28 days, the time between ovulation and the next period is about two weeks, so progesterone is measured about seven days before the expected period, or on Day 21. However, if a woman’s cycle is longer or shorter than 28 days, the testing day will be adjusted accordingly. For example, a woman with a 35-day cycle would be tested for progesterone on Day 28.
If you are not ovulating, there are steps that can be taken to help release the eggs, including drugs. Your fertility specialist will discuss these options with you.
An ultrasound exam is also used to measure your uterine lining to determine if it is thick enough for a fertilized egg to implant.