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How Do Home Pregnancy Tests Work?
All pregnancy tests, including home pregnancy tests, detect a hormone that is present only when a woman is pregnant. This hormone, called human chorionic gonadotropin or HCG, is also called the pregnancy hormone.
Types of Tests
There are two types of pregnancy tests: blood and urine tests. Home pregnancy tests use urine and can determine pregnancy about 2 weeks after ovulation, or about the time you miss a menstrual period. Many women choose home pregnancy tests because they're inexpensive and easy to use. When a woman has a positive result, she needs to make an appointment with her health care provider, however, to verify her pregnancy and begin prenatal care.
Procedure
Each home pregnancy test has its own instructions that you must follow carefully in order to get the most accurate result. All involve either collecting urine and dipping a stick into it, or holding a stick under the urine stream. After the urine is in contact with the testing materials, there's a waiting period, usually a few minutes, until you can read the test. Each test is read differently, too -- one might have a stripe for a positive result, for example, and another might have a plus sign. In addition to the written instructions, most tests also have toll-free phone numbers to call if you have any questions about how to do the test or read the results.
Accuracy
Home pregnancy tests are very accurate. Although they typically claim to be 99 percent accurate by the day you miss your period, MayoClinic.com disagrees with this claim: "Although research suggests that most home pregnancy tests don't consistently spot pregnancy this early, home pregnancy tests are considered reliable when used according to package instructions one week after a missed period."
Accuracy can also vary with actual use. If you follow the instructions carefully, take an accurate reading and are sure the test is not past its expiration date, you will tend to get more accurate results. Most brands of home pregnancy tests tell users to do the test again in a few days, no matter what the results.
If you use a test too early in your pregnancy, you may not have enough of the pregnancy hormone in your urine to have a positive test result. You can also get a negative test result if you are not pregnant or if you ovulated later than you thought you did. If your test is negative, test yourself again in a week. If you keep getting a negative result and think you are pregnant, talk with a health care provider.
Drug Interactions
Most medications, both over-the-counter and prescription drugs, including birth control pills and antibiotics, should not affect the results of a home pregnancy test. Only those drugs that have the pregnancy hormone HCG in them, which can be used for treating infertility, can give a false positive test result. Alcohol and illegal drugs do not affect pregnancy test results, either, but you should not be using these substances if you are trying to get pregnant.
