How accurate are over-the-counter pregnancy tests?
The modern home pregnancy tests are now very accurate. Although each test brand may differ somewhat, the basic principle behind how they work is the same—the test measures the amount of hCG, human chorionic gonadotropin, a pregnancy hormone, in your blood.
You may be anxious to know the minute that you become pregnant—and some tests may claim to let you know that early—still, you may get a false-negative for a variety of reasons. For example, if you didn't follow the package directions exactly, or if you urine was diluted from drinking several glasses of water. So if you question the results of a home pregnancy test try taking the test again—and even again a week later if you suspect that you're pregnant but still getting a negative result.