If you think you might be pregnant, chances are you already took a home pregnancy test or visited a local clinic. Whether you got a positive or negative test result, right about now, you might be wondering whether that result was accurate. How common are false negative pregnancy test results? False positives? What if there’s just a faint second line? Can you really trust what that tiny result screen is telling you? Can a pregnancy test be wrong? Let’s take a closer look at pregnancy test accuracy.
How do pregnancy tests work?
To understand the accuracy and possibility of false-positive pregnancy tests or false-negative pregnancy test results it helps to understand how a pregnancy test kit works.
Home pregnancy tests detect a hormone called Human Chorionic Gonadotropin (hCG), also known as the pregnancy hormone. hCG ensures progesterone production continues to maintain your uterine lining during pregnancy. Cells that will later form your placenta immediately begin releasing this hormone after the fertilized egg implants in your uterine lining.
While a blood test can detect any level of hCG, most home urine tests have a threshold of about 20-25 mIU/mL. However, the most sensitive home test (First Response Early Result) can detect levels as low as 5.5 mIU/mL. Either way, these levels are typically reached until 3 to 5 days after implantation. You can read more about when to take a pregnancy test for the most accurate results here.
How common is a false negative pregnancy test result, and why do they happen?
By definition, a false negative pregnancy test is a test that says not pregnant when you actually are pregnant. False-negative pregnancy test results are actually rather common in early pregnancy as they have numerous causes.
For instance, a retrospective cohort study performed by researchers at the Boston University School of Medicine in 2021 found that among 10,924 urine hCG tests performed during emergency room visits, 1.6% yielded a false negative. Rates were higher among women suffering from ectopic pregnancy (3.6%), a condition where a fertilized egg implants outside the uterus, such as in the fallopian tube or ovary, as hCG levels are typically lower than normal in these cases.
Keep in mind these rates were in a hospital setting with the instructions for completion and test results read by a trained medical professional. There currently are no studies on the prevalence of false-negative results in the real world, but as you can imagine, they are much higher.
You may receive a false negative pregnancy test result if:
-You tested too early, and hCG levels were not yet sufficient.
-You didn’t test too early, but hCG levels were still insufficient.
-Your hCG levels were too high (the hook effect).
-Your urine was too diluted.
-The test was done incorrectly. For example, perhaps the test was not saturated entirely in urine.
-The test was expired.
-The test results were read too long after testing.
-You misread the result (1 in 4 women are said to misread traditional pregnancy tests).
If you think you may be pregnant or are experiencing symptoms of pregnancy and get a negative pregnancy test result, it’s best to test again in around a week. If the second test is negative, but symptoms persist (including missed periods), you might consider seeing your health care provider. In the meantime, you might also find our page on missed periods with a negative test helpful.
How common are false positive pregnancy test results, and why do they happen?
False-positive pregnancy tests are far less common because a true false positive result (not a misread test) is harder to obtain, but they can happen. As an example, in the substantial equivalence laboratory report for First Response’s Gold Digital Pregnancy Test, which included 575 tests, only .18% (a single test) yielded a false-positive. These tests feature an easy-to-read digital result screen that shows a plus sign and yes for positive results or minus sign and no for negative results. In the review, 98% of test-takers read their test result correctly, meaning the .18% is a fair representation of the prevalence of real false-positive test results.
You may receive a false positive pregnancy test result if:
-You misread the result. Some tests will leave a very faint line as the urine passes the test indicator window that can appear to make the test positive. Tests that come back with very faint results should be re-done in a few days to a week.
-You got an evaporation line. Some tests, usually the cheaper variety, will show a “ghost line,” which is typically colorless or thin even if you are not pregnant. Reportedly among conception circles, this is more common in blue-dye tests. If you have to squint to see the line, test again in 48 hours.
-You waited too long to read the results. In most cases, an aged result will appear negative, but there have been cases where they turned from negative to positive after the recommended read time. This line is more likely to be colorless.
-You were pregnant but aren’t anymore. If a pregnancy fails within days of implantation, hCG is still produced. This is called a chemical pregnancy. Chemical pregnancies often present with no other symptoms, unlike a miscarriage, which often comes with heavier bleeding and cramping at the least. Early pregnancy loss is common, occurring in roughly 1 in 4 pregnancies.
-You are taking a drug that contains hCG or causes analytical interference in hCG testing. A few select fertility medications, such as , contain hCG, but a longer list of common medications can interfere with the accuracy of hCG testing, including anticonvulsants, like Carbamazepine, and antipsychotics, like Chlorpromazine and Thioridazine. Speak to your pharmacist about any medications you may be taking before you attempt to use a home pregnancy test. Note that birth control can not cause a false positive.
-You have cancer or an ovarian cyst. Certain medical conditions, including gestational trophoblastic diseases (molar pregnancy), breast, ovarian, and lung cancer, and ovarian cysts, can release hCG, which could result in a false positive.
-You are post-menopausal or just got a really sensitive test. The female body naturally contains low levels of hCG (which are higher in some post-menopausal women), and the accuracy of pregnancy tests varies. In very rare cases, a test will read positive based solely on the normal hCG levels in your body.
Can you trust home pregnancy tests?
A home pregnancy test is a good first step in determining if you are or aren’t pregnant if you miss a period or are experiencing other pregnancy symptoms. Keep in mind, though, they aren’t without fail, making them a better first step than last. When in doubt, see a health care provider, especially if what caused that doubt continues.