Spotlight: Ovulation Predictor Kits

How to use OPKs to chart your cycle and predict fertile days.

Kristina Nickerson

Ovulation Predictor Kit (OPK)

Reliability: High

When to monitor:

  • From the end of menstruation until ovulation is indicated.
  • We do not recommend using OPKs after ovulation.

An ovulation predictor kit, or OPK, is a stick that you urinate on exactly like a pregnancy test. They are denoted with a blue label in most cases, while HCG (the hormone released during pregnancy) tests are denoted with a pink label in most cases. The lines in an OPK bind to Leutinizing Hormone (LH). LH triggers the release of a mature egg and surges about a day and a half before ovulation in most people. Some, more advanced and more expensive, kits also have a site that binds to estrogen. Estrogen also surges before ovulation, but more slowly. The slow rise of estrogen can show up to 5 days before ovulation, giving more warning.

How to do it:

Each brand of kit, and each type of kit, can have slightly different instructions. So make sure to follow the instructions with your kit. In general, you should urinate on an OPK stick in the mid afternoon and at the same time every day. This should be done the week after bleeding or the week you normally ovulate, and should be stopped once ovulation is indicated. You can keep testing throughout your cycle, but there are a couple of reasons not to do this. The easiest to understand is that OPKs can be expensive, especially if you’re using ones that also test estrogen or using digital tests. The second reason is that there is a second LH (and estrogen) surge just before menstruation that can be confusing, especially if you listen to the internet. Having a second surge does not indicate pregnancy. It is a normal thing during any given cycle so please don’t use it to test early.

Record the day your LH surge is indicated. You can use graph paper or an app.

How to interpret it:

There are two lines on an OPK, one line is the test line and one is the control line. Your particular kit will tell you which is which. When the test line becomes darker than the control line, you have reached your LH surge and should expect ovulation within a day or so, or whenever you normally ovulate after your surge. The key word here is “darker”. It is not when it is as dark, only when it is darker.

One of the drawbacks of an OPK is that it only gives you about 24 hours notice before ovulation, and you really should have sperm started on their journey the three days before ovulation for your best chances at pregnancy. Another drawback of OPKs is that they don’t work well with certain fertility drugs or for women in peri-menopause. LH can be artificially increased in both cases. Lastly, they don’t show ovulation, only that your body has signalled your ovary to drop an egg. There is no guarantee that your ovary will actually do it. For these reasons OPKs should be used in conjunction with at least basal body temperature and be used to establish when in your cycle you ovulate for future reference rather than used to plan your reproductive sex for that particular cycle.


The photo below shows an ideal OPK progression. Yours may or may not look like this, but the important thing to take away is that the bottom test is the positive test. This person likely ovulated 24-36 hours after they took that test.


For best results, you should urinate on the OPK at the same time every day.

Testing in mid afternoon is likely to catch your surge earlier than testing in the morning.

Buy the cheap sticks off the internet, they are just as accurate as something more expensive, as long as you remember darker.