The Myth of the Day 21 Progesterone Blood Test

Have you attended your doctor to have a 'Day 21 Progesterone blood test' carried out? Have you left feeling more confused by the result than you were before?

If you read my first post this week on the timing of female fertility blood tests you will understand why your result may be incorrect.

Here's a reminder...remember this graph?shutterstock_296962172 (2)png

Now carefully follow the hormone Progesterone which is Yellow in colour. At the start of the cycle Progesterone is at very low levels. After Ovulation (Day 14 on this graph) the levels start to rise and peak on Day 21. In this graph pregnancy does not occur so Progesterone levels fall rapidly after Day 21 and a new cycle begins on Day 28.

What is Progesterone?

Progesterone is a hormone which is produced by the ruptured follicle after Ovulation. At this stage the follicle is called the Corpus Luteum. (Hence the name Luteal phase for this part of the cycle). Progesterone prepares the lining of the uterus for implantation. If implantation occurs the level of Progesterone continues to rise. If not, the levels of Progesterone decrease and as a result the lining is shed, marking the start of a new cycle.

How long does the Luteal Phase last?

The length of your Luteal Phase should be the same each month and should never vary by more than 1 to 2 days. If your Luteal Phase is short (<10 days) an embryo does not have enough time to implant before the lining is shed. If this occurs you may have a Luteal Phase Defect. You may require additional Progesterone to maintain your lining and allow a pregnancy to get established.

Why is my Day 21 Progesterone result incorrect?

As you can see from the graph, the rise and fall of Progesterone happens quite quickly. If you had your Progesterone blood test taken on Day 21 but you do not have a regular 28-Day cycle then your result may be incorrect. For most women, their cycles do not follow the typical 28 Day pattern shown above. A normal menstrual cycle lasts anywhere from 23 to 35 days.

Your cycle is unique to you. Your body reacts to your own hormones in a certain way. The key to figuring out your cycle is to be able to identify the subtle clues your body sends out each day. You can do this by tracking your Basal Body Temperature, monitoring your cervical mucus or  using Ovulation Predictor Kits.

Your Progesterone blood test should be performed 7 days post Ovulation or 7 days before the start of your next cycle.

Here are a few examples

If you normally have a 24 Day cycle and you Ovulate on Day 10, the most accurate day to have your Progesterone checked is on Day 17. By Day 21 your Progesterone level will have dropped if you are not pregnant so you will get an abnormally low result.

If you normally have a 34 Day cycle and you Ovulate on Day 19 then the most accurate day to have your Progesterone checked will be Day 27. If you check your Progesterone on Day 21 of this type of cycle, you will get a very low result as the levels have not had a chance to rise post ovulation.


If your cycle is not regular then it may take some time to get an accurate Progesterone reading. You may need to do this over a number of months. A much easier way to determine if your levels of Progesterone are rising is to carefully track your cycle using Basal Body Temperature. A rise in Progesterone increases Basal Body Temperature.

As I mentioned above, counting the number of days in your Luteal Phase can also identify if you have a problem with Progesterone.

Test your Progesterone at Home

You can perform your own Progesterone Blood test at Home using a simple fingerprick blood test. This test is fast and easy to do and I will have your results in just 3 DAYS. You will receive a full laboratory report and if you wish you can arrange a phone consultation with me FREE OF CHARGE to discuss the result.

No Doctors appointments or time off work required and you will always do the test on the correct day of your cycle.

Order Your Home Blood Test Here!/Progesterone-at-Home-Blood-Test/p/74401891/category=0


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