Could you be pregnant? Get answers to common questions about home pregnancy tests.

By Mayo Clinic Staff

Taking a home pregnancy test can be nerve-wracking, 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. You're likely to get more accurate results, however, if you wait until after the first day of your missed period or, better yet, one week after your missed period.

Why wait? Shortly after a fertilized egg attaches to your uterine lining (implantation), the placenta forms and produces the hormone human chorionic gonadotropin (HCG). This hormone enters your bloodstream and urine. During early pregnancy, the HCG concentration increases rapidly — doubling every two to three days. The earlier you take the home pregnancy test, the harder it might be for the test to detect HCG. Keep in mind that the exact timing of ovulation might vary among women or even from month to month, and the fertilized egg can implant in the uterus at different times. This can affect when HCG production begins and becomes detectable.

If it's important to confirm your pregnancy right away, depending on how far along you are in your pregnancy, your health care provider might recommend that you have an ultrasound, repeat a urine test in the hospital or clinic lab, or have a blood test to measure your HCG.

With most tests, you place the end of a dipstick in your urine stream or immerse the dipstick in a container of collected urine. A few minutes later, the dipstick reveals the test result — often as a plus or minus sign, one line or two lines, or the words "pregnant" or "not pregnant" on a strip or screen.

However, some home pregnancy tests are more sensitive than others. In other words, in some tests the amount of HCG needed to be detected in the urine to produce a positive test result is lower.

Always check the test's expiration date and read the instructions carefully before you take the test.

Many home pregnancy tests claim to be 99 percent accurate. However, research suggests that many home pregnancy tests are not sensitive enough to diagnose pregnancy in women who have recently missed a period. For the most reliable results, wait to take the test until one week after your 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, menopause or problems with your ovaries 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. 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 ultrasound 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 menstrual periods (amenorrhea), including thyroid disorders, low body weight, problems with your ovaries, excessive exercise and stress. If you're not pregnant, your health care provider can help you get your menstrual cycle back on track.
Dec. 02, 2015