How do ovulation tests work? (2025)

What is an ovulation test kit?

An ovulation test strip (or ovulation predictor kit, also known as an OPK) can help you identify the most fertile days during your monthly cycle – that is, the day or two leading up to ovulation when sex (or insemination) is most likely to lead to pregnancy. It can be a quick and easy way to predict when you're about to ovulate.

You don't have to schedule sex in order to get pregnant. Having sex every few days during the middle two weeks of your cycle will usually do the job. But you may want to be more strategic and pinpoint your most fertile days if:

• You and your partner have hectic schedules or already have children, and it's not feasible to have sex that often.
• You're trying to get pregnant through insemination.
• You have irregular cycles that make it hard to find your fertile window.
• You've had difficulty conceiving after a few months of trying.

How do ovulation test strips work?

There are two kinds of ovulation prediction kits:

Urine-based OPKs

These test your urine for an increase in luteinizing hormone (LH). This usually happens within a day and a half before ovulation. There's always a small amount of LH present in your blood and urine, but the level will go up by about two to five times in the days before ovulation.

A test stick usually shows a positive result about 24 to 36 hours before your egg is released, so plan to have sex (or be inseminated) during that window if you want to maximize your chances of getting pregnant.

Salivary ferning kits

Salivary ferning kits help you tell when ovulation is imminent by showing changes in your saliva. As your estrogen levels rise in the days before ovulation, the salt content of your saliva increases.

If you put a drop of saliva on a glass slide, the salt may dry and crystallize into a fern-like pattern that you may be able to see with a pocket-size portable microscope. The ferning pattern should indicate that you'll ovulate in the next few days (though you may continue to see a ferning pattern up to two days after ovulation).

What does a positive ovulation test mean?

A positive result means that you're likely to ovulate in the next 24 to 36 hours, so if you're trying to conceive, try to have sex more frequently in the next several days (or schedule your insemination ASAP, if that's the approach you're taking).

A positive test result isn't a guarantee, though, and all test kits have different parameters for displaying a positive result. Make sure you've read the instructions for your individual kit and know how to interpret the results.

"If you have been using OPKs and not getting pregnant, seek a fertility specialist within a year, or within 6 months if you're over 35," says Tarun Jain, M.D., a board-certified reproductive endocrinology and infertility (REI) physician and member of BabyCenter's Medical Advisory Board. "Keep in mind that it can sometimes take a few months to get an appointment, so ask your ob-gyn for an early referral."

How accurate are ovulation test kits?

Urine-based LH tests are more accurate than salivary ferning kits, but they're not foolproof. Follow the instructions on your kit carefully for the most accurate results. Make sure you use the test at the time of day recommended, and read the results within the timing window specified.

Also, keep in mind that because LH can surge with or without the release of an egg, the tests can't tell you for sure whether you've ovulated.

Most importantly, don't use these kits to try to avoid pregnancy, since you won't know exactly when you've ovulated or when your fertile window has closed for that cycle.

Salivary ferning tests are less accurate than urine tests. Ferning may happen as early as six days before you ovulate as well as at other times in your menstrual cycle, particularly if you're taking the fertility medication Clomid (clomiphene).

It may also be hard to recognize whether ferning has happened on the test slide. If you have poor eyesight, salivary ferning kits may not be the best method for you.

Taking Clomid or drugs containing human chorionic gonadotropin (hCG) or LH can affect results for both salivary ferning and urine-based tests. Ask your provider whether you need to stop taking certain drugs before using these tests.

Finally, OPKs are not likely to be accurate for women who are nearing menopause or have polycystic ovary syndrome.

How to use ovulation kits

First and foremost, always read and follow the instructions that come with the kit you select.

Whatever kind of kit you're using, you'll need to figure out which day of your cycle to start testing. Some kits suggest that you count back 18 days from the day you expect your next period. So if you have a 28-day cycle, start testing on day 10 (the 10th day after your period starts), and continue until you get a positive result.

As far as how to use the kit and how to read your test results, the steps will differ based on whether you're using a urine-based or salivary ferning OPK.

Using a urine-based OPK

Urine-based OPKs supply five to 20 days' worth of test sticks. Once a day, you hold a test stick in your urine stream or dip the end of the stick into urine you've collected in a cup. The colored bands or symbols that appear on the test stick indicate whether the LH surge is occurring.

Try to collect your urine at about the same time every day, but follow the instructions on your particular kit for best results.

Don't drink a lot of liquid during the two hours before testing. Too much liquid dilutes your urine, which could make it more difficult to detect the surge.

Read the results within 10 minutes. A positive result won't disappear, but some negative results may later display a faint second color band that would be misleading.

Using a salivary ferning OPK

Use a finger or lick the slide to put a little bit of your saliva on a slide. Do this first thing in the morning, before you've had anything to eat or drink. Make sure the sample is free of air bubbles.

Wait for the saliva to dry and then use the microscope to see whether there is any ferning. Compare your slide with examples in the instructions to tell how to identify ferning.

Where to get ovulation test kits

You can buy kits online as well as over-the-counter at most drugstores and supermarkets (they're usually near the pregnancy tests).

The more challenging thing about getting a hold of ovulation test kits is affording them. Urine-based OPKs cost between $20 and $50 and contain between five and 20 test sticks. Most brands offer the same level of reliability, so pick the one that offers you the most test sticks for the least amount of money.

Once you detect your surge, you can stop testing for that cycle and save any unused test sticks for the following month (unless you conceive, of course).

Salivary ferning OPKs can be a better value. After the initial investment of about $30 for the microscope, you should be able to test again and again. If it takes you a long time to conceive, you may have to replace the kit with a new one after about two years, depending on the brand.