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 pregnancy test out of the trash hours later, because miracles can happen, right? And suddenly, you see not one, but two lines potentially confirming your pregnancy status. Can it be? What happens if you leave a pregnancy test too long? Does leaving your pregnancy test out for too long cause a false positive?
Will a pregnancy test turn positive after sitting?
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, infertility specialist, tells Romper.
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 Noorhasan. “This happens when you leave the test out too long before reading it.”
How do home pregnancy tests work?
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.
Each brand of home pregnancy test offers different estimates for how long the results are good for, so be sure to be mindful of how many minutes you’ve got depending on which brand you’re using. Some require you to read the results upon completion of the test, which is what Noorhasan also advises. “Most urine pregnancy tests kits will have you read the stick within three minutes of applying the urine,” says Noorhasan. “The best advice I can give to avoid an error is to follow the instructions in the test package.”
More reasons you may see a false positive
Also, it’s worth noting that 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 two-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 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 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.
Dr. Dorette Noorhasan, MD, a board-certified Reproductive Endocrinologist, infertility specialist and author of The Fertility Manual: Reproductive Options for Your Family
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