When you take an at-home pregnancy test, no matter what result you're hoping for, you want it to be accurate. That's why it's so alarming that a company is recalling more than 58,000 pregnancy tests in the U.K. over concerns they might give a false positive result—meaning the test could tell you that you’re pregnant when you’re really not.
The company, Guangzhou Wondfo Biotech, recently announced that it is recalling one lot of Clear & Simple Digital Pregnancy Tests after they produced a “small number of false positive results,” a spokesperson for the U.K.’s Medicines and Healthcare products Regulatory Agency (MHRA) tells SELF.
People who may have purchased this item are advised to check and see if they have one of the affected tests, which have the catalog number DM-102, lot number DM10220170710E, and expiration date of January 2020. “This fault is limited to one lot and has now been resolved,” the MHRA spokesperson says. Any consumers who have the tests should return them to the place where they bought them for a refund.
At-home pregnancy tests work by detecting the presence of human chorionic gonadotropin (hCG), which your body produces during pregnancy.
After a fertilized egg attaches to your uterine lining, the placenta (which helps nourish and maintain the fetus during pregnancy) forms and makes hCG, the Mayo Clinic explains. That hCG is then excreted into your urine, where it can eventually be picked up on a urine test, Melissa Goist, M.D., an ob/gyn at The Ohio State University Wexner Medical Center, tells SELF.
During the first trimester, your hCG level usually doubles every 72 hours. About 12 to 14 days after you conceive, your hCG level is usually high enough to be picked up by at-home urine tests, according to the American Pregnancy Association.
Many at-home pregnancy tests have you pee on a stick to try to detect hCG in your urine. That urine then travels up a strip where it flows into a control area or window, Jamie Alan, Ph.D., Pharm.D., an assistant professor of pharmacology and toxicology at Michigan State University, tells SELF. (This is that line that shows up whether you’re pregnant or not.) If you have hCG in your urine, it triggers a chemical reaction that will cause another line, plus sign, or “pregnant” symbol to appear on your test.
Similarly, digital tests have a sensor inside that detects the chemical change that happens when you have hCG in your urine, Alan says.
In the case of these faulty digital tests, there was a large gap between the test strip bracket and the test’s plastic casing, which interfered with the sensor, according to the the MHRA spokesperson. This caused the sensor to read a positive when there actually wasn’t one.
False positives aren't common, but there are a few reasons (other than a faulty test) why you might get one.
At-home pregnancy tests are generally pretty reliable—most at-home pregnancy tests claim to be about 99 percent accurate, the Mayo Clinic says. But, as SELF explained previously, there are some reasons why you might get an inaccurate reading.
If you let the test sit too long before you look at it, you can see an evaporation line that could be mistaken for a positive, Alan says. Every test is different, but most recommend waiting between three and five minutes to check the results.
A test could also give you a false positive if it’s expired. “Home pregnancy tests, like medications, have an expiration date,” Gerardo Bustillo, M.D., an ob/gyn at MemorialCare Orange Coast Medical Center in Fountain Valley, Calif., tells SELF. “All pregnancy tests last approximately two to three years once produced.” (Check the label before you use a test, just to be sure it’s still good.)
You could also get a false positive if you're using fertility drugs that raise your hCG levels, Alan notes. If you test too soon after taking the medication, it may be detected in your urine and give you a false positive. After you give birth or have a miscarriage, it can take four to six weeks for hCG levels to go back down, the American Pregnancy Association says. So testing too soon afterward could also give you a false positive. You could also have what’s known as a chemical pregnancy, which is essentially a very early miscarriage and can also cause a false positive, Alan says.
And, in rare cases, you could have an ovarian tumor that secretes hCG, Jessica Shepherd, M.D., a minimally invasive gynecologist at Baylor University, tells SELF.
If you get a positive pregnancy test result, you should check in with your doctor, Dr. Shepherd says.
They’ll want to see you in a few weeks and will run their own urine test and possibly a blood test that also detects hCG to confirm the pregnancy, she explains. And, if you’re not sure if it’s positive or you got a faint line, wait a few days and test again. Above all else, don’t hesitate to call your doctor if you’re not sure what’s going on.