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Fertility Testing

Day 21 Testing 


On Day 21 of your cycle, your fertility specialist may want to check certain hormone levels, like progesterone and estradiol (E2) in your system, and measure the thickness of your endometrium (uterine lining).


What is Tested?

Here's a quick hormone lesson: When you ovulate, a follicle in the ovary releases an egg into the fallopian tubes, where it then travels down into the uterus. After the release, the follicle left behind in the ovary releases a large amount of progesterone. 

Therefore, when you are tested on Day 21, approximately 7 days after ovulation, the 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 a month (normal ovulation produces only one egg a 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 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 testing 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 flow, not including any spotting.

What Happens After Day 21 Testing?

If the Day 21 testing results show that you are not ovulating, there are steps that can be taken to help release the eggs, including drugs. Your reproductive endocrinologist will discuss these options with you.

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