You pee on a stick, and you wait. And wait. Those two minutes can feel like an absolute eternity when you’re hoping for a positive pregnancy result. So when you see one lonely line on the test, you might feel a little let down. But then you fish the test out of the trash later, because miracles can happen, right? And suddenly, you see not one, but two lines potentially confirming your pregnancy status. Can it be? Or does leaving your pregnancy test out for too long cause a false positive?
The short answer is a resounding yes. It is possible for a pregnancy test that’s left out for too long to show a false positive.
“We tell patients to avoid taking a urine pregnancy test since sometimes the results can be inaccurate,” Dr. Dorette Noorhasan, a board certified Reproductive Endocrinologist and infertility specialist and author of The Fertility Manual: Reproductive Options for Your Family, tells Romper.
But first, a primer on pregnancy tests. They work by detecting the presence of human chorionic gonadotropin (hCG). This hormone is found both in the blood and urine of a pregnant woman, and home pregnancy tests can detect it as early as two weeks after conception, according to the American Pregnancy Association.
The major reason why a false positive test result can appear after you’ve taken a pregnancy test has to do less with hCG levels and more with your pee. If you’re using a urine test, that second line which you might think is proof-positive that you’re pregnant is actually an evaporation line left from your urine. “The urine can evaporate leading to false positive results,” explains Dr. Noorhasan. “This happens when you leave the test out too long before reading it.”
Medical News Today reported that home pregnancy tests are 99 percent effective when used correctly, but that’s the critical factor. It means that you have to both follow the instructions down to the letter the first time and read the test results within the allotted time period.
Each brand has different estimates on how long the results are good for, so be sure to be mindful of how many minutes you’ve got. Some require you to read the results upon completion of the test, which is what Dr. Noorhasan also advises. “Most urine pregnancy tests kits will have you read the stick within three minutes of applying the urine,” says Dr. Noorhasan. “The best advice I can give to avoid an error is to follow the instructions in the test package.”
There are other instances in which you can have a false positive pregnancy test result. If you had a prior miscarriage or abortion, your hCG levels might still be high enough to render the wrong result, Healthline reported. Also, fertility medications and other prescriptions that contain hCG and are used to treat various medical conditions, such as Parkinson’s, can produce a false positive.
Of course, you could just have a defective test, which is why it’s always a good idea to purchase a 2-pack package, just in case. And always check the expiration date on any test you might be buying, since an older test might yield inconsistent results. “The chemical to detect the hCG hormone may not work well and hence lead to a false positive,” notes Dr. Noorhasan.
Still, it’s better to be safe that sorry, so if a positive appears well after the test’s completion, you might want to follow up by taking another pregnancy test in a week or so. “If you have a positive pregnancy test, you should contact your doctor’s office so they can order a blood pregnancy test to confirm the results,” states Dr. Noorhasan. That way, you’ll know for sure if you’re truly pregnant.
Time doesn’t move any slower than when you’re waiting those few minutes for a pregnancy test result. But time is of the essence when it comes to reading them. Make sure to follow the test’s instructions and read the response right away. After all, those two little lines just might begin the next beautiful chapter of your life.