Using Ovulation Test Strips to Detect Your Most Fertile Time

Ovulation Predictor Kits, How They Work, and How to Use Them

Pros and cons of ovulation predictors

Verywell / Cindy Chung  

Ovulation test strips are urine-based tests that you use at home to detect impending ovulation. They work by detecting the hormone LH. Sometimes they are called OPKs, ovulation predictor kits, or simply ovulation tests.

When you buy an ovulation test kit, it comes with several tests strips (narrow thin papers), or they may look more like pregnancy test sticks. You either pee on the extended tip of the pregnancy-test-like version, or you pee in a cup and then carefully dip the test strip into your urine.

The results can tell you if you might be ovulating soon. (More on reading the results below.)

Using ovulation strips can help you time sex for pregnancy. When you get a positive result on the test, you should have sex every day for the next two or three days.

What to Buy

There are a variety of ovulation predictor kits available. Clearblue Easy and First Response are the most popular.

Like pregnancy tests, you can pay relatively little or quite a lot, depending on how much technology you want. The most expensive variety of ovulation predictors is digital.

The Clearblue Easy Fertility Monitor is one of the more popular digital tests. This monitor detects two hormones, LH, and estrogen. This allows it to detect more fertile days than the paper tests. 

The cheapest ovulation tests are simply strips of paper. You can purchase the test strips online in bulk, but they often come with no instructions for use.

What you choose to buy and use has more to do with your comfort and your ease in reading the test strips. They aren't always easy to interpret.

A digital monitor takes the guesswork out of ovulation testing. A digital test will tell you whether you're fertile or not. That said, it is pricey.

The paper tests, especially the cheaper ones, aren't simple to read. Unlike a pregnancy test where you either have a line or you don't, an OPK test requires you to determine if the test line is darker than the control line. That's not always easy to determine. (More on this below.) 

Despite the slightly more difficult result interpretation, some people love the extra cheap test papers.

How Predictor Kits Work

Ovulation predictor kits work by detecting the levels of LH hormone in your urine. LH stands for luteinizing hormone. As ovulation approaches, LH spikes in order to push the egg into the final stages of maturity. This spike of LH is called the LH surge. About 36 hours after the LH surge, ovulation occurs.

To increase your chances of getting pregnant, you should have sex the two days before ovulation occurs. Since OPKs detect the LH surge that occurs 12 to 36 hours before ovulation, you can be sure to have sex at just the right time for conception.

The Clearblue Easy Fertility Monitor detects LH and estrogen. Because estrogen begins to rise before the LH surge, the Clearblue Easy Fertility Monitor can give you more warning that ovulation is coming. This will allow you to have conception sex for up to a week before you ovulate.

How to Use a Kit

Be sure to read the instructions of your particular ovulation predictor kit, since there may be slight variations on how they work.

Generally, however, an ovulation predictor kit comes with a set of test strips or sticks. Some OPKs come with five tests, others with as many as 10.

You should begin using the tests about two days before you expect to ovulate. If you’re not sure when you ovulate, you can use an ovulation calculator or the chart. There are also fertility apps that will attempt to guess at when ovulation is likely to occur for you. 

If your cycles are irregular, you should test according to the earliest and latest dates you’d expect to ovulate. It helps to have a kit with several test strips if this is your situation.

Ovulation predictor kits have two lines. One line is the control line. This just lets you know that the test was used properly and is working. The second line is the test line. When the test line is as dark or darker than the control line, LH is surging.

This is when you should start having baby-making sex.

If you test for five days, you have an 80% chance of predicting ovulation. If you test for 10 days, you have a 95% chance of predicting ovulation.

Clearblue Easy Fertility Monitor works differently from regular ovulation predictor kits. You need to start testing on the first day of your period.

Whatever ovulation test kit you use, be sure to read the instructions carefully for the best results.

Advantages and Disadvantages of OPKs

  • Relatively easy to use. Unlike charting your body basal temperature, you don’t have to remember to have a thermometer right by your bed or avoid moving too much when you wake up.

  • Unlike BBT charting, they tell you when ovulation is approaching, not that it's already passed. Unless you're also checking your cervical mucus, a BBT chart can’t tell you when you should have sex.

  • Ovulation kits don’t need to be used right when you get up. Morning urine is best, but taking the test within the same six-hour window every day should give accurate results.

  • If you're not comfortable checking for cervical mucus, you may feel better using the test strips.

  • Using one month after month can get expensive. This is especially so if your cycles are irregular and you need to use more than five test strips.

  • Some people have trouble reading a positive ovulation test result. If you don’t get a very strong LH surge, it can be difficult to tell whether the test line is as dark as the control line.

  • Ovulation tests may not work well for women with polycystic ovarian syndrome, who can have several LH surges or high levels of LH throughout their cycles.

  • Since ovulation kits test for LH, they may yield positive results all the time or on several days. These positive results are false-positives—they don't mean you're ovulating.

  • They can signal that your body is trying to ovulate, but they can't confirm that ovulation happened. It's possible for LH to surge and an egg to never release.

While ovulation predictor kits cannot confirm that ovulation actually took place, body basal temperature charting can.

Ovulation predictor kits can be used alongside other methods of ovulation detecting, like BBT charting. This can give you more assurance and help you get to know your body better.

You may want to use an ovulation predictor kit when you just start fertility charting for added confidence. Once you get the hang of charting your BBT and cervical mucus, you can drop the expensive ovulation test strips.

For women who find fertility charting stressful, though, ovulation predictor kits can be a great way to predict ovulation and time sex for pregnancy.

Analyzing the Results

Let’s say you go through your ovulation test strips, and you never get a result indicating ovulation. Why might this happen?

One possible reason is you started testing too early in your cycle.

Let’s say you have an ovulation kit that contains five tests and you started testing on Day 13 of your cycle. Day 17 would be your last test day. But if you don’t ovulate until Day 25, for example, you may not get a positive result because you weren't testing on your most fertile days, Day 22, 23, and 24.

Another possibility is you started testing too late. Let’s say you ovulated on Day 12, but didn’t start testing until Day 14. In this situation, you may miss the LH surge.

This is one reason it helps to have an idea of when you tend to ovulate depending on the length of your cycle. The longer your cycles are, the more likely it is that you ovulate later than average. The shorter your cycles are, the more likely it is that you ovulate earlier than average.

You can find a helpful chart on this page that will tell you the best time to start testing, depending on your cycle length.

Another possible reason you won’t get a positive result is you’re not ovulating. It’s not abnormal to have one off cycle, once in a while. However, if you don’t get a positive result after a few months, or if your cycles are irregular, talk to your doctor.

Early Pregnancy

You may have heard that ovulation tests can be used to detect pregnancy. The answer is yes, they can! But... they are nowhere near as accurate as a pregnancy test.

The reason that ovulation tests can act as a pregnancy test (kind of) is that the hormone LH (that ovulation tests detect) is molecularly similar to hCG (the pregnancy hormone that pregnancy tests detect.)

So theoretically, if you’re pregnant, and you use an ovulation test, you may get a positive result. However, it’s also very possible for you to be pregnant and for an ovulation test to not return a positive result. You might think you’re not pregnant when you really are. Pregnancy tests are more reliable. 

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3 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Kerin J. Ovulation detection in the human. Clin Reprod Fertil. 1982;1(1):27-54.

  3. Cole LA. Biological functions of hCG and hCG-related molecules. Reprod Biol Endocrinol. 2010;8:102. doi:10.1186/1477-7827-8-102

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