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Fertility Testing

What is Day 21 testing?

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). It is an important step in the fertility testing process.

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What does Day 21 testing include?

Here's a quick hormone lesson:

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. 

Therefore, when you undergo testing on Day 21, approximately 7 days after ovulation, your clinician 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.

Additionally, estradiol (E2) is included in the Day 21 testing. This is a form of estrogen that is released as your egg follicles grow in the ovary. It peaks at ovulation.

Serial estradiol (E2) levels are often measured for monitoring superovulation in intrauterine insemination (IUI) and in vitro fertilization (IVF) treatment cycles, as it indicates follicular growth. Superovulation is the controlled stimulation of the ovaries as they are medically manipulated to produce more than one egg each month (normal ovulation produces only one egg each month).

An ultrasound exam is also used to measure your uterine lining to determine if it is thick enough for a fertilized egg to implant.


Why is this testing done on Day 21?

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 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 test day will be adjusted accordingly. For example, a woman 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).

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What happens after Day 21 testing?

If your Day 21 testing results show that you are not ovulating, don't worry! There are steps that can be taken to help release the eggs, including fertility medications. Your reproductive endocrinologist will discuss these options with you to find the best option for you.

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