Day 21 Progesterone Testing: What it is & Why it’s Important

Having trouble getting pregnant – or staying pregnant? You may want to check your progesterone. 

Progesterone is nicknamed the ‘hormone of pregnancy’ for good reason. It helps the uterus become an ideal environment for growing a healthy baby. 

So if your progesterone isn’t high enough after fertilization, you’ll have a tough time getting pregnant.

Taking a progesterone test can give you a peek at your levels. That way you know if they’re in the right range for pregnancy.

But to get an accurate gauge, you need to time it right. And day 21 is the magic number. 

Read on to learn what normal progesterone levels are on day 21 and why testing that day is key.

What is day 21 fertility testing?

Day 21 fertility testing is a crucial part of assessing a woman’s fertility. The main focus of day 21 testing is to check your progesterone levels, although other hormones like estrogen may be tested as well.

So why day 21? Well, a textbook menstrual cycle is roughly 28 days long. Halfway through, on around day 14, ovulation occurs. That’s when one of your ovaries releases an egg. 

About seven days after ovulation (or day 21 if you have a 28-day cycle), progesterone levels peak. And progesterone plays a key role in helping you get pregnant.

But is day 21 the best day for testing progesterone? Read on to know further.

What is progesterone?

Progesterone is a female sex hormone made by your ovaries, and in smaller amounts, by your adrenal glands. Like estrogen, it plays a vital role in pregnancy and helps regulate your menstrual cycle. 

Estrogen is the dominant hormone for the first half of your cycle (the follicular phase). But progesterone is the star in the second half (the luteal phase). 

The length of the follicular phase varies between 10 to 16 days. But the luteal phase is more predictable, lasting 14 days.

After you ovulate, the ruptured follicle that holds the egg becomes the corpus luteum. And this temporary structure’s job is to produce progesterone. This helps thicken the uterine lining and make it sticky so a fertilized egg can attach. 

Progesterone keeps rising before peaking at 6-8 days past ovulation (DPO). 

If a fertilized egg doesn’t implant in the uterus, progesterone drops, and along comes your period. But if implantation is a success, progesterone continues to rise. 

Normal progesterone pattern

Progesterone doesn’t rise in a steady stream in the luteal phase. It rises in pulses. That’s why your progesterone levels can fluctuate day to day – and even hour to hour. 

Early in the luteal phase, progesterone’s rise is relatively stable. But progesterone levels become more erratic in the mid to late luteal phase. 

In fact, research shows progesterone levels can fluctuate as much as 8-fold within 90 minutes! That’s why you can’t base your fertility on day 21 on a single progesterone test. 

Why should you get your progesterone levels tested?

There are two main reasons…

Reason #1: You’re having trouble getting pregnant

If you’ve been trying to conceive without any luck, you may not be ovulating. This is known as anovulation. It can happen due to hormonal imbalances, irregular cycles, or polycystic ovary syndrome (PCOS) to name a few. 

If your doctor suspects you’re not ovulating, they’ll likely suggest a progesterone test. That’s because progesterone production depends on ovulation. If you don’t ovulate, your progesterone levels will suffer. 

On the flip side, if your progesterone rises in the luteal phase, that’s a surefire sign ovulation occurred. 

But as you know, successful ovulation is just step one in getting pregnant. To get that beloved big fat positive, a sperm needs to fertilize the egg – and it needs to implant in the uterine wall. 

Implantation happens around 6 to 12 DPO, but 8 to 10 DPO is most common. And progesterone plays a crucial role in this process. It thickens the endometrium, so the fertilized egg can stick. 

To get pregnant, your progesterone needs to rise after ovulation and stay elevated long enough for implantation to occur. Research shows low progesterone levels after ovulation increases the risk of implantation failure.

Checking your progesterone levels can help you find out if they’re in the right range to support implantation.

Read More: How to Make Implantation Successful: What You Need to Know 

Reason #2: To monitor your pregnancy

Progesterone helps you get pregnant – and stay that way! That’s because one of progesterone’s main jobs is to maintain the uterine lining, which is crucial for pregnancy.

Studies show low progesterone levels increase the risk of miscarriage. Keeping tabs on your levels can give you peace of mind that you have enough progesterone to support ongoing pregnancy. 

And if your pregnancy is considered high-risk, tracking your progesterone can help you monitor your progress.

Read More: The Importance of Progesterone Production During Early Pregnancy 

How is a progesterone test done?

You can check your progesterone levels with a blood test or urine test. While blood testing is the most accurate method, it requires a lab visit. This can be inconvenient, not to mention pricey. 

Another affordable, more convenient option is to use the Inito Fertility Monitor. Inito tracks the progesterone metabolite PdG in your urine from the comfort of your home. You even get hormone charts of your actual values to share with your doctor. 

Read More: PdG Test: Key Things To Know About Progesterone Tests 

When to test for progesterone?

Progesterone levels are typically tested on day 21 of your cycle. Hence, the name ‘Day 21 fertility testing’. 😉 This is because progesterone peaks around 7 days after you ovulate. 

But that is true only for the ones who have a textbook 28-day cycle. If you have a 34-day cycle, your ovulation day is 14 days before your next period. That would be around day 20 of your cycle. And your progesterone levels would peak at 7 days after ovulation, that is day 27!

Plus, there are some downsides to taking a single test. 

As mentioned, progesterone levels fluctuate throughout the day. So say you take a blood test in the morning. If you test again that afternoon, you could get completely different results!

That’s why you can’t get an accurate gauge of your progesterone from one test.

If you’re only checking your progesterone to confirm ovulation, one test may be enough. Studies show that a random blood progesterone level of ⩾5 ng/ml will confirm ovulation. 

In this case, you can test between 2-7 DPO of your cycle. If your progesterone rises during this period, it means you ovulated. 

But if you’re testing your progesterone to see if it’s in a healthy range to support pregnancy, you’ll need more than one test. That’s because progesterone levels need to stay elevated for 6-12 days after ovulation to support implantation. 

Read More: What Do Your Progesterone Levels Mean After Ovulation?

What are normal progesterone levels on day 21?

Ideally, at day 21 you want your blood progesterone levels to be 10 ng/mL or higher. This ensures the best chance of successful implantation and pregnancy. So if your progesterone levels meet that mark, you can rest easy. 

Research shows women with progesterone levels below 5.6 ng/mL are less likely to get pregnant. If you’re trying to conceive and find your progesterone levels are low, it’s important to talk with your doctor.

Read More: Low Progesterone: All You Need to Know

Progesterone testing

As mentioned, progesterone levels fluctuate throughout your cycle and even day to day. So your results can differ greatly based on your cycle day.

Check out the chart below for blood progesterone levels:

Menstrual cycle phase

Normal progesterone levels (ng/mL)

Follicular phase

0.1-0.7 ng/mL

Luteal phase

2-25 ng/mL

If you test on day 21 in the mid-luteal phase and your results show low progesterone levels, it can be confusing and frustrating. And there are a few reasons that can happen.

Here are four scenarios where you may see a low day 21 progesterone level:

  • You tested before ovulation. If the follicle hasn’t ruptured yet, then your progesterone levels will stay low. Testing before ovulation may happen due to irregular cycles.
  • You tested right after ovulation. Progesterone levels don’t shoot up right away. It takes time. Give them a chance to rise and retest in a week.
  • You tested right before your period. If an egg isn’t fertilized, your progesterone levels drop. This prompts your period to start. This is most likely to happen if you have shorter cycles. 
  • You didn’t ovulate. It’s estimated that a third of all menstrual cycles are anovulatory. And without ovulation, your progesterone will stay low. 

What if my progesterone levels at day 21 are not normal?

Well for starters, take a deep breath – because you may have tested on the wrong day. 

Yes, you’ll have peak progesterone levels on day 21 if you have a textbook 28-day cycle. But only around 16% of women have 28-day cycles on the regular. 

If your cycle is shorter or longer than 28 days (which let’s face it, most are), then you’ll need to adjust when to test your progesterone. 

If you have shorter cycles, you’ll need to test earlier. And if you have longer cycles, you’ll want to test later. Let’s understand better…

Generally, you ovulate 12-14 days before your next period arrives. 

For example, if you have a 21-day cycle, you’ll ovulate on day 7. In this case, you wouldn’t hit peak progesterone levels until a week later, on day 14. And if an egg isn’t fertilized, you’d get your period on day 21.

On the other hand, say you have a 35-day cycle. You’d ovulate on day 21, which means your peak progesterone wouldn’t hit until day 28. 

As you can see, timing the test right is everything. 

When should I test if I have irregular cycles?

If your cycles are irregular and you’re confused about when to test, one easy hack is to track your ovulation. But you’ll need an ovulation test that measures your LH and progesterone. Here’s why…

Luteinizing hormone (LH) surges 24-36 hours before you ovulate. This helps predict when ovulation is coming – but it doesn’t confirm it. For that you need progesterone. 

If progesterone rises soon after the LH surge, that’s proof you did release an egg. Once progesterone is detected, simply wait 7 days to retest. By then your progesterone should be at peak levels. 

Inito measures your LH and PdG, a metabolite of progesterone, as well as estrogen and follicle stimulating hormone (FSH). This helps you track your fertile window and confirm ovulation despite irregular cycles. 

Use Inito to track your PdG levels at 6-8 DPO and confirm ovulation.

How to improve your progesterone levels?

If you find your progesterone levels are on the low side, don’t worry. Making a few simple shifts to your diet may be all it takes to boost your progesterone. 

Your body depends on certain nutrients to make progesterone. These include vitamin C, vitamin B6, zinc, and magnesium. Eating more foods rich in these nutrients can support healthy progesterone levels. 

To support progesterone production, increase your intake of the following foods:

  • Foods high in vitamin C: bell peppers, kiwi, strawberries, oranges, tomatoes, & broccoli
  • Foods high in vitamin B6: chicken, turkey, beef, salmon, tuna, eggs, chickpeas, sweet potato, carrots, spinach, & bananas
  • Foods high in zinc: beef, pork, chicken, shellfish, mushrooms, pumpkin seeds, cashews, lentils, oats, & yogurt
  • Foods high in magnesium: almonds, cashews, pumpkin seeds, dark chocolate, bananas, avocados, black beans & spinach

Keeping a lid on stress is also crucial for progesterone production. That’s because progesterone is actually the precursor for the stress hormone cortisol. 

When you’re stressed out, your body ramps up cortisol production. This can, in turn, reduce your progesterone levels. 

So do what you can to manage your stress. Even setting aside as little as 10 minutes for daily stress relief can help. Journal, meditate, take a walk, or do some deep breathing. Whatever helps you relax. Your progesterone levels will thank you.

Read More: How To Increase Progesterone To Get Pregnant? 

When to see a doctor?

If you’re eating a healthy diet and keeping stress at bay, but your progesterone is still low, talk with your doctor. They can help identify any underlying issues that may be disrupting your hormones. Your doctor may also recommend progesterone supplementation to help you get pregnant.


  • Progesterone nourishes the uterine lining to help a fertilized egg attach. 
  • If you have a 28-day cycle, progesterone peaks at day 21.
  • Day 21 progesterone testing can help assess how likely you are to get pregnant. 
  • Ideally, on day 21 progesterone should be at least 10 ng/mL for successful implantation.
  • You can take a progesterone blood test or urine test to see if your levels are in a normal range.
  • Progesterone fluctuates and rises in pulses. So one progesterone test isn’t enough to get a clear picture of your levels. 
  • Testing on day 21 won’t work unless your cycle lasts 28 days. If your cycles are longer or shorter, you’ll need to test seven days after you ovulate.
  • You can use Inito to track your fertile window and confirm your ovulation. That way you know the right time to check for peak progesterone metabolite PdG levels. 

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