Home pregnancy tests work by detecting a hormone called beta-HCG, which is an abbreviation for beta-human chorionic gonadotropin. Beta-HCG is only present if a woman is pregnant. Because some of the hormone enters the urine, home pregnancy tests can detect it there.
Manufacturers of home pregnancy tests advertise that their tests as being very accurate in determining pregnancy–“99.7% accurate,” for example. The true accuracy of a home pregnancy test, however, may depend on how far along the pregnancy is and how well the user is able to interpret the test result.
Because the beta-HCG hormone levels may be low during early pregnancy–during the time before a period is missed and for about 5 days thereafter–the tests are less accurate during that time period. Once a woman is more than 5 days “late” for her period, the accuracy of the test increases, but it’s still relatively low: The range is anywhere from 70 to 90%. The test is most accurate once a woman is more than 20 days late.
If you think you may be pregnant and are having any difficulties with home pregnancy tests, talk to your doctor. He or she can provide a sensitive pregnancy test and provide information about how to keep you and your developing baby healthy.