Should You Test Progesterone Levels on Day 21 of Your Cycle?
- Progesterone testing should only be done on day 21 of the cycle if ovulation occurs around day 14
- The correct time to test progesterone is at the halfway point of the luteal phase
- Recent research calls into question whether progesterone blood tests are even capable of identifying low progesterone levels
If you’re having trouble conceiving, one of the hormones your doctor may want to test is progesterone. The conventional wisdom is that your progesterone levels should be at their highest on day 21 of your cycle.
It turns out that this conventional wisdom is sometimes wrong. Testing hormone levels on day 21 of your cycle only makes sense if you ovulate on (or close to) day 14. If your doctor brings up day 21 testing without first confirming your usual cycle length and ovulation day, you should clarify a few things. Here’s everything you need to know to have a productive conversation with your doctor.
Why does your doctor check progesterone levels?
Your doctor may want to check your progesterone levels for two reasons:
- To confirm that you are ovulating. Ovulation is the only way that your body can produce progesterone (more on that below).
- To confirm that if you are ovulating, that you are producing enough progesterone.
What is progesterone anyway—and why is it important?
Progesterone is a very important hormone for sustaining pregnancy. It maintains the uterine lining, ensuring that it’s a friendly environment for an embryo to implant.
Before you ovulate, high levels of estrogen cause your uterine lining to grow. You should have very low levels of progesterone in your system at this part of your cycle.
After you ovulate, progesterone starts to rise, reaching a peak about midway through your luteal phase. Adequate progesterone during this phase of your cycle ensures that the endometrium (uterine lining) transforms into a receptive state.
When is the best time to check progesterone levels?
That depends on when you ovulate. The day 21 blood test is supposed to check your progesterone levels at the time when they should be at their peak. In a 28-day cycle with ovulation on day 14, progesterone levels will be at their peak around day 21.
But what if you don’t ovulate on day 14? If you ovulate, say, on day 22, then a day 21 blood test will show very low levels of progesterone. In this case, a day 29 blood test will give you a much more accurate picture of your true progesterone levels.
What should progesterone levels be?
What is considered normal for progesterone levels varies depending on what phase of you’re menstrual cycle you’re in:
0.1 – 0.7 ng/mL during the follicular stage (before ovulation)
2 – 25 ng/mL during the luteal stage (after ovulation)
However, one thing to keep in mind is that blood tests may not be the best diagnostic tool for low progesterone levels or luteal insufficiency. Progesterone is released from the pituitary gland in “pulses,” meaning that progesterone levels can vary widely (up to 8-fold!) over the course of a few hours.
Because of this fluctuation, a single blood test value is of limited value in diagnosing low progesterone levels. Some researchers have suggested using three serial blood tests in a row as a better diagnostic tool, but even with this method it’s difficult to get precise results.
A 2015 study concluded that it was “remarkable” that isolated progesterone blood tests are still used to diagnose low luteal phase progesterone levels, “despite the clearly established barriers to its use”.
How to talk to your doctor
Hopefully, you have a great doctor who works with you to make sure that progesterone testing is done at the appropriate time in your cycle. But if your doctor recommends day 21 testing without any information about when you ovulate, it’s time to advocate for yourself.
If you’re already tracking your cycle, then you should absolutely tell your doctor when you ovulate and how long your luteal phase usually is. If you’re not tracking your cycle, then consider using the temperature method just for the cycle when your doctor wants to test your progesterone levels, so you can make sure the test is done at the right time.