Will an Ovulation Test Be Positive If You’re Pregnant?

It is one of the most searched questions of the two-week wait: you have a box of ovulation strips, and you wonder if they can tell you what you really want to know. They can react to pregnancy, sometimes. Here is why, why it is unreliable, and what else can turn an ovulation test positive after ovulation.

Will an ovulation test be positive if you're pregnant?

It can be, but often it will not be, and you cannot tell which from the strip. Pregnancy makes human chorionic gonadotropin (hCG). An ovulation test is built to detect luteinizing hormone (LH). The two hormones are so similar that some LH tests react to hCG when there is enough of it. Whether yours does, and at what level, depends on the antibodies in that brand's strip, and no published study has measured how often ovulation tests turn positive in early pregnancy.

Why hCG can trigger an ovulation test

LH, FSH, TSH and hCG are all built from two pieces, an alpha and a beta subunit. The alpha subunit is the same in all of them.² What makes each hormone different is the beta subunit, and the beta subunits of hCG and LH share 85% of their sequence over their first 114 amino acids; hCG's has an extra tail.¹

Immunoassays, including test strips, work by antibodies grabbing the hormone. With that much shared structure, antibodies raised against one hormone can bind the other: researchers have shown that antibodies to hCG can neutralize LH because the two share many binding sites.³ Ovulation test makers choose antibodies to target LH, but that does not guarantee they ignore hCG at high levels.

Why it is not a reliable pregnancy test

Early hCG is low. In daily urine data from 109 women who conceived, median hCG was about 12 mIU/mL at 10 days past ovulation, 25 at 11 days and 137 at 14 days.⁴ Most ovulation strips are set to detect LH at 25 mIU/mL or more, and only if hCG cross-reacts strongly would the test line reach control-line darkness. Around the missed period, results are hit and miss.

The reading rules are different. On an ovulation test, a faint test line is normal and means negative, because some LH is always present. On a pregnancy test, any line in the time window usually means positive. A faint line on an LH strip in the two-week wait tells you nothing about pregnancy. See how to read an ovulation test.

They are not built or reviewed for it. Ovulation tests are Class I devices exempt from FDA premarket review, while home pregnancy tests are Class II and need 510(k) clearance, with data on how accurately they detect hCG.⁶⁷ Nothing in an ovulation test's design or labeling supports using it to diagnose pregnancy. Pregnancy tests, by contrast, are checked for the reverse problem: the FDA files for First Response Early Result and Clearblue Early Digital report that LH at 500 to 1,000 mIU/mL did not affect their results.¹²¹³

Other reasons for a positive ovulation test after ovulation

A dark LH strip a week or more after ovulation has several possible causes:

• Early pregnancy hCG, as above.
• LH that rises more than once. In an analysis of 283 cycles with ultrasound-confirmed ovulation, LH crossed 25 mIU/mL and fell back before ovulation in 31% of cycles, so LH does not always follow a single clean curve.⁵
• Raised baseline LH, as in PCOS. In one study, 75% of women with PCOS who had not recently ovulated had raised LH levels.⁸
• Fertility medicines. Clomiphene increases LH.⁹ An hCG "trigger" injection puts hCG in your urine, which can turn both ovulation and pregnancy tests positive; pregnancy test makers note that medicines containing hCG can cause a false positive.¹⁰
• Misreading a faint line, or reading after the time window.

What to do instead

Take a pregnancy test. The earliest a sensitive one is likely to work is about 9 to 10 days past ovulation; a reliable result usually takes 12 to 14 days. Our guide to how many days after ovulation you can test shows hCG levels day by day, and the DPO calculator finds your test date.

The FDA advises treating a negative as tentative and retesting after several days if you still think you may be pregnant.¹¹ Pregnancy strips cost as little as $0.25 each, so there is no saving in using an ovulation strip instead.

If you are still trying to time ovulation next cycle, see when to take an ovulation test and how to read an ovulation test.

What if your period is late and the ovulation test is positive?

Take a pregnancy test now. By the time a period is late, hCG in a continuing pregnancy is usually well above 25 mIU/mL: in daily urine data the median was about 137 mIU/mL at 14 days past ovulation and over 200 the day after.⁴ Any FDA-cleared pregnancy test should be able to see that. If the pregnancy test is negative and your period still does not come, retest in a few days, and talk to your doctor if your period is more than a week late; late ovulation is a common explanation.

If your ovulation tests are positive on many days of most cycles, talk to a doctor about checking for PCOS or other causes of high LH. If you get a positive pregnancy test followed by bleeding, or severe one-sided pain, shoulder pain or fainting, seek care the same day; these can be signs of an ectopic pregnancy. More on the two-week wait in the Wait step.