Taking a pregnancy test can be a nerve-wracking experience. But how do you know you can trust the results when the test comes back positive? Although relatively rare, there are some things that can cause a false positive pregnancy test. In other words, the test will read as positive when you aren’t really pregnant.
For the most part, though, at-home pregnancy tests are highly accurate. “Pregnancy tests, even those at the grocery store, are incredibly accurate, about the same as a urine test you would take at your doctors office, around 99%,” as Dr. Nicole Williams, OB/GYN & Founder of The Gynecology Institute of Chicago, tells Romper. In most cases, you can trust the accuracy of that positive (or negative) test result, but there are a few factors that can skew the results. First, though, it’s helpful to understand a bit more about how pregnancy tests work and how to take them to ensure the most accurate results. (Yes, there’s a whole art to peeing on a stick.)
How do pregnancy tests work?
So how does a pregnancy test read your urine? “They test for the hormone human chorionic gonadotropin, which is the hormone made in pregnancy, and is seen as early as your first missed period,” says Dr. Williams. Testing for human chorionic gonadotropin, also known as hCG, is one of the earliest ways to determine a pregnancy, and there are even hCG calculators that can help show how your pregnancy is progressing.
When to take a pregnancy test
Timing matters. “The best time to take a pregnancy test is when your period is actually considered late — the day after your expected period date,” as Dr. Jessica White-Videa, DO, FACOG, Board Certified OB/GYN at Northwest Medical Center, tells Romper. “Also, it is best to take a pregnancy test first thing in the morning when urine is most concentrated.”
How to read a pregnancy test
It’s also crucial to know how to read a pregnancy test correctly, because the exact instructions can vary depending on which brand you use. “Make sure you’re reading the test correctly and that you know how to use the at-home test. Inaccurate use of the test can also create a false positive scenario,” as Dr. Taraneh Shirazian, OB/GYN and Brand Founder of Mommy Matters, tells Romper. “Repeat your test with different pregnancy kit brands to ensure that the test is persistently positive.”
What causes a false positive pregnancy test
Here’s what the doctors have to say about the reasons your pregnancy test’s positive result may not be correct. (Again, this is a relatively rare occurrence.)
Also known as a chemical pregnancy, a biochemical pregnancy can also potentially skew the results of your pregnancy test. “Pregnancy tests these days are much more sensitive to the pregnancy hormone, hCG, and thus even low amounts of hCG may result in a positive home pregnancy test,” says Dr. Zore. “That test may then turn negative, or you may get a heavy period a few days after your expected period followed by a negative test. These are known as biochemical pregnancies.”
Put another way, a chemical pregnancy is “a very early unrecognized pregnancy which has already begun to deteriorate” by the time you test, as Dr. Williams explains.
“Certain medical conditions, such as chronic renal disease, pituitary abnormalities or even some forms of cancer can lead to a rise in your hCG levels and lead to a false positive pregnancy test,” says Dr. Zore.
“Certain medications, such as those used in assisted reproductive technology and even some antidepressants,” can cause a false positive reading, says Dr. Williams. This is particularly true for medications that contain hCG, the hormone that pregnancy tests are made to detect. “This may be seen in cases of women who recently went through fertility treatment and took a shot with hCG in it to ‘trigger’ ovulation,” as Dr. Temeka Zore, OBGYN/REI and Modern Fertility Medical Advisor, tells Romper. “Many times these medications, like novarel, pregnyl, or ovidrel, contain hCG. If you take a pregnancy test within a week of injecting this medication, you may get a false positive pregnancy test if it’s still lingering in your system.” Talk to your doctor if you’re concerned about any current medications potentially interfering with your pregnancy test results.
Misreading / Misusing the test
Honestly, reading all those faint lines can get pretty confusing, and the tests can be super sensitive. In fact, even leaving a pregnancy test out too long can cause a false positive result. “Many pregnancy tests have a minimum and maximum amount of time the test should be read by to ensure the best results. If you happen to take a pregnancy test, forget about it and then recheck it outside of the testing window, it is possible an evaporation line will appear as a faint second line,” says Dr. Zore. Expired tests can also give incorrect results, as Dr. Zore further explains.
Even being very eager to read the results may lead to an inaccurate reading. “False negative results can be caused by checking a urine pregnancy test too soon,” says Dr. White-Videa.
“A recent miscarriage or abortion may continue to give you a positive pregnancy test,” says Dr. Shirazian. Your hCG levels may still be elevated. On a more personal level, trying to conceive after a miscarriage can be an emotional journey, so don’t hesitate to reach out to a loved one or counselor for support.
Working With Animals
Consider your workplace. “If you work as a vet or near farm animals, there is a certain antibody you may develop that looks like pregnancy on blood tests,” says Dr. Williams.
Although at-home pregnancy tests are generally accurate in their results, there are a few factors that can cause a false positive reading. If you have any questions about your pregnancy test or conception choices in general, don’t hesitate to reach out to your OB/GYN for answers.
Dr. Taraneh Shirazian, OB/GYN and Brand Founder of Mommy Matters
Dr. Jessica White-Videa, DO, FACOG, Board Certified OB/GYN at Northwest Medical Center
Dr. Temeka Zore, OBGYN/REI and Modern Fertility Medical Advisor