Day 21 Testing: The Importance Of Day 21 Fertility Testing


If you are trying to conceive, or are starting IVF treatment, your doctor may want to carry out a day 21 fertility test to check the levels of progesterone your body is producing.

We look at everything you need to know about this essential part of the IVF process in our guide to the importance of day 21 fertility testing.

What is the Day 21 Progesterone Test?

This test is conducted to check the levels of progesterone in your system as well as the thickness of the endometrium, or uterine lining.

It is called the Day 21 Test as it is completed on the 21st day of your menstrual cycle, which is around seven days before your next period. The process involves a blood test so that your fertility consultant can assess your hormone levels and identify any potential causes of reduced fertility.

Day 21 testing is often done in conjunction with a Day 3 or FSH test, which is done on the third day of a period and measures three other important hormones – Follicle Stimulating Hormone (FSH), Luteinizing Hormone (LH), and oestradiol. A Day 3 test aims to assess whether you are ovulating and gives an indication of your egg supply.

What does it aim to show?

By taking it on day 21 of your menstrual cycle – when your progesterone should be at its peak – the test aims to show that your levels are normal and that you are ovulating. A test result that returns a low day 21 progesterone will indicate that no egg has been produced and that natural fertilization could not happen.

Why are progesterone levels so important?

When it comes to ovulation and sustaining a pregnancy, progesterone is vital as it works to maintain a healthy uterine lining and create the right environment for an embryo to implant. If you do not have enough of the hormone, the lining of your uterus may start to shed before implantation, making it harder for you to get pregnant.

What should my progesterone levels be?

The level of progesterone in your system will fluctuate during your menstrual cycle. Before you ovulate, high levels of oestrogen will stimulate the lining of your uterus, causing it to grow. And it is during this part of the cycle that there should be low levels of progesterone in your system.

However, after you ovulate, the levels of progesterone should start to rise, peaking midway through your luteal phase, where an egg is released. The raised levels of progesterone during this phase will ensure the lining of your uterus is in a receptive state, ready for an embryo to implant.

When it comes to actual levels of progesterone every woman is different but as an indication of ‘normal’ ranges, pre-ovulation should be less than 1 nanogram per milliliter (ng/mL), while mid-cycle it should rise to between 5 to 20 ng/mL.

How a 21 Day Test can help your treatment plan

If your 21-day test reveals low or fluctuating progesterone levels, then your consultant will discuss with you the best way forward to address any issues of potential infertility that are indicated by the results. As well as medication to help you ovulate, other possible treatment options include IVF to increase your chances of a successful pregnancy.


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