If you’ve spent any time at all on a fertility forum, you’ve probably come across this frequently asked question: can ovulation tests (also known as ovulation predictor kits, or OPKs) be used to test for pregnancy?
The short answer is yes, it’s possible. If you’re pregnant and you take an ovulation test, it will become positive. However, we don’t recommend it, for reasons we’ll discuss below.
Why you will get a positive ovulation test if you’re pregnant
Ovulation tests are designed to detect the peak of LH (luteinizing hormone) which occurs right before ovulation. The female body produces some LH at all times through the cycle, though, which is why there’s always a visible test line on OPKs.
Pregnancy tests, on the other hand, detect hCG (human chorionic gonadotropin). This hormone is only produced during pregnancy (and by some cancerous tumors, but that’s another story).
Now, here’s the thing: the molecular structures of LH and hCG are very similar—in fact, nearly identical.
Ovulation tests don’t distinguish between the two, because the molecules they detect are common to both hormones. If you have either LH or hCG in your urine, the test will detect it. And if there’s a high enough amount of one or the other, the test becomes positive.
This is why ovulation tests will be positive if you’re pregnant.
There is a small difference between the two hormones, however. See those extra N’s and O’s on the image representing hCG? (They’re called sites of N-linked and O-linked glycosylation, by the way, but you really don’t need to know that!)
That’s what pregnancy tests are looking for: the tiny part of the structure that’s unique to hCG. No matter how much LH there is in your urine, pregnancy tests will not turn positive unless they also find hCG.
Therefore, ovulation tests can be used as pregnancy tests, but not the other way around.
Digital ovulation tests as pregnancy tests
It doesn’t matter what kind of ovulation test you use: strip test or midstream test, digital test or standard. They all react to both LH and hCG.
Never take digital tests (ovulation tests or pregnancy tests) apart and try to decipher the strip inside them. The strip will almost always have two lines, but this doesn’t mean anything. You can’t interpret the lines like you would a standard test. Trust the result you got in the digital window.
Why we don’t recommend using OPKs as HPTs
So why do we need pregnancy tests at all, if ovulation tests do the job just as well?
While it does work most of the time, there are several reasons why we don’t recommend using OPKs for pregnancy testing.
Most importantly, a positive OPK can be a source of confusion. A positive pregnancy tests leaves no doubts: you’re pregnant indeed. A positive ovulation test, on the other hand, could mean that you’re pregnant, but it could also mean that for some reason, there’s lots of LH in your urine.
Interpreting the test can be tricky, too.
With pregnancy tests, again there’s no doubt: a line is a line. If there’s a visible test line, you’re pregnant. Ovulation tests, on the other hand, will almost always have a visible test line, and the test is only positive if the test line is as dark as the control line, or darker.
So what’s going on if the test line is darker than usual, but not quite as dark as the control line? Or if they’re almost the same shade? Are you pregnant? Maybe, maybe not …
Finally, ovulation tests are typically less sensitive than pregnancy tests. If you’re actually pregnant, chances are you’ll get a positive HPT before the OPKs turn positive.
In conclusion: if you already know you’re pregnant, by all means have fun with your spare OPKs. But for actually detecting pregnancy, stick to pregnancy tests.