Inaccurate Pregnancy Test Results: False Negative and Positive Causes

If you think you may be pregnant, the first place you likely thought to head is to the store to buy a home pregnancy test. However, knowing if you are or aren’t pregnant is pretty important information. This leads many to wonder: how common are false negative pregnancy test results? False positives? Can you really trust what that tiny result screen is telling you? Can a pregnancy test be wrong?

How do pregnancy tests work?

False Negative Pregnancy Test Results: Chances and CausesUnderstanding the accuracy and possibility of false positive pregnancy tests or false negative pregnancy test results depends on understanding the way a pregnancy test works.

Pregnancy tests detect a hormone called Human Chorionic Gonadotropin (hCG) also known as the pregnancy hormone. hCG facilitates the continuance of progesterone production that maintains your uterine lining during pregnancy. This hormone can usually be detected about 10 to 12 days after conception as the cells that will later form your placenta begin to release it immediately after a fertilized egg has implanted in your uterine lining.

How common is a false negative pregnancy test result, and why do they happen?

False negative pregnancy test results are actually rather common as they have numerous causes. A false negative pregnancy test by definition is a test that says not pregnant when you actually are pregnant. While most tests are in the 97 to 99 percent accuracy range, this doesn’t account for misuse or malfunction of the test.

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 causes of missed periods other than pregnancy helpful.

How common are false positive pregnancy test results, and why do they happen?

False positives on a pregnancy test are far less common. This is because a true false positive, excluding a misread of result, is harder to obtain, but they can happen.

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. 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 naturally aborts early on, hCG is still produced. This is called a chemical pregnancy.

-You are taking a drug that contains hCG. Some fertility drugs in particular contain hCG. Be sure that any and all medication you may be taking doesn’t contain hCG if 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 kinds of cancer and ovarian cysts can release hCG as well, 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 levels of hCG 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.

You may also find helpful:
Can you have a period and be pregnant?
Early Signs of Pregnancy Quiz
DIY Pregnancy Tests: Are They Accurate?

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