On Day 21 of your cycle, your fertility doctor may want to check levels of certain hormones in your body, like progesterone and estradiol (E2), and measure the thickness of your endometrium (uterine lining).
This is an important step in the fertility testing process that provides valuable information about your reproductive health. The results of your Day 21 testing can help inform your next steps and increase your chances of success with fertility treatments such as IVF or IUI.
When you ovulate, a follicle in your ovary releases an egg into the fallopian tubes, where it then travels down into the uterus. After the egg is released, the follicle left behind in your ovary releases a large amount of progesterone.
When you undergo Day 21 testing (approximately 7 days after ovulation), your doctor should see a surge in progesterone. A high Day 21 progesterone level indicates ovulation and the release of an egg. A low Day 21 progesterone level suggests the cycle was anovulatory (no egg was produced). If no egg is produced, pregnancy cannot be achieved.
An ultrasound exam is also used to measure your uterine lining to determine if it is thick enough for a fertilized egg to implant.
The timing of ovulation is associated with a peak in progesterone, and that spike is related to the subsequent menstrual period - not the preceding one.
In an average cycle (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.
If your cycle is longer or shorter than 28 days, the test day will be adjusted accordingly. For example, someone with a 35-day cycle would be tested for progesterone seven days prior to the subsequent menstrual cycle, on Day 28.
When calculating your Day 21, make sure you're starting from a true Day 1, which is the first day of full flow (don't include spotting).
If your Day 21 testing results show that you are not ovulating, don't panic! There are steps that can be taken to help release your eggs, including fertility medications, and a low progesterone level does not necessarily mean you won't be able to get pregnant.
Your reproductive endocrinologist will discuss all of your options with you to find the best path forward and help you achieve your goals.
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