(MayoClinic.com) Taking a home pregnancy test can be nerve-racking, especially if you're not sure whether you can trust the results. Know when and how to take a home pregnancy test — as well as some of the possible pitfalls of home testing.

Many home pregnancy tests claim to be accurate as early as the first day of a missed period — or even before. For the most reliable results, however, wait until one week after a missed period.

Shortly after a fertilized egg attaches to your uterine lining, the hormone human chorionic gonadotropin (HCG) begins production in the placenta and enters your bloodstream and urine. During early pregnancy, the HCG in your blood increases rapidly — often doubling every two to three days. Many home pregnancy tests can reliably detect this hormone in your urine one week after a missed period. Days earlier, however, some home pregnancy tests might not be as precise.

If it's important to confirm your pregnancy right away, ask your health care provider about a blood test to detect HCG. The blood test, which can be done in most health care providers' offices, is more sensitive than is the urine test.

Various types of home pregnancy tests are available. With most tests, you place the end of a dipstick in your urine stream or immerse the dipstick in a container of collected urine for five to 10 seconds. A few minutes later, the dipstick reveals the test result — often as a plus or minus sign, a line or lines, a color change, or the words "pregnant" or "not pregnant" on a strip or screen.

Keep in mind that instructions might vary from kit to kit. Read the instructions carefully before you take the test. If you have questions about how to take the test or interpret the results, contact the manufacturer. Look for a toll-free number or the manufacturer's website in the package instructions.

Many home pregnancy tests claim to be 99 percent accurate on the day you miss your period. Although research suggests that some 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.

Fertility drugs or other medications that contain HCG might interfere with home pregnancy test results. However, most medications, including antibiotics and birth control pills, don't affect the accuracy of home pregnancy tests.

Although rare, it's possible to get a positive result from a home pregnancy test when you're not actually pregnant. This is known as a false-positive.

A false-positive might happen if you had a pregnancy loss soon after the fertilized egg attached to your uterine lining (biochemical pregnancy) or you take a pregnancy test too soon after taking a fertility drug that contains HCG. An ectopic pregnancy or menopause also might contribute to misleading test results.

It's possible to get a negative result from a home pregnancy test when you're actually pregnant. This is known as a false-negative — and it's much more likely to occur than is a false-positive. You might get a false-negative if you:

• Take the test too early. The earlier after a missed period that you take a home pregnancy test, the harder it is for the test to detect HCG. For the most accurate results, take the test one week after a missed period — when the level of HCG in your urine is most likely to be detectable. If you can't wait that long, ask your health care provider for a blood test.
• Check test results too soon. Be sure to give the test time to work. Consider setting a timer according to the package instructions.
• Use diluted urine. Drinking too much fluid before taking a home pregnancy test might cause a false-negative result. For the most accurate results, take the test first thing in the morning — when your urine is the most concentrated.

Based on your test results, consider taking the following steps:

• Your home pregnancy test is positive, or you've taken a few home pregnancy tests and gotten mixed results. Make an appointment with your health care provider. You might need a blood test or pelvic exam to confirm your pregnancy. The sooner your pregnancy is confirmed, the sooner you can begin prenatal care.
• Your home pregnancy test is negative. If your period doesn't begin, repeat the test in a few days or one week — especially if you took the test before or shortly after a missed period.
• You continue to get negative test results, but your period doesn't begin or you still think you might be pregnant. Check with your health care provider. Many factors can lead to missed periods, including illness, strenuous exercise, weight loss, stress and hormonal imbalances. If you're not pregnant, your health care provider can help you get your menstrual cycle back on track.

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