A False Negative Pregnancy Test
If a woman suspects she's pregnant, she might first take a home pregnancy test. These tests measure the level of a hormone called human chorionic gonadotropin (hCG) in urine. Elevated levels of this hormone suggest a woman is pregnant.
But home pregnancy tests can give false readings if used improperly, read wrong or taken too early. Women who don't know they're pregnant may have taken a pregnancy test and thought their results were negative when they were actually positive. This is also called a false negative pregnancy test. The tests themselves can also be confusing, so keep the instructions nearby just in case. Although these tests claim to be 99 percent accurate, they don't always pick up on all women's hCG levels in the first days of pregnancy.
Health experts suggest waiting at least one week after a missed period to take a home pregnancy test. Even then, it might be a good idea to use more than one test for reassurance.
Most medications don't influence the accuracy of pregnancy tests, but in some cases, fertility treatments that include hCG can produce a false positive test, or one that reads pregnant when the woman isn't [source: WomensHealth.gov].
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