Taking a home pregnancy test can be exciting, but it also may be stressful. That's especially true if you're not sure whether you should trust the results. Know when and how to take a home pregnancy test. And learn some of the possible drawbacks of home testing.

Many home pregnancy tests claim to be able to tell whether you're pregnant as early as the first day of a missed period. Some claim you can take them even before that point. But home pregnancy test results are more likely to be accurate if they are taken after the first day of a missed period. That's because shortly after a fertilized egg attaches to the uterine lining — a process called implantation — the placenta begins forming. The placenta makes the pregnancy hormone human chorionic gonadotropin (HCG). HCG can be found in blood and urine. HCG is present in the body only during pregnancy.

A home pregnancy test checks to see if there's HCG in the urine. During early pregnancy, the amount of HCG in blood and urine rises quickly — doubling every 2 to 3 days. That means if you wait a day or two after your missed period to take the test, and you are pregnant, it's more likely the test will find HCG and show a positive result.

The timing of ovulation makes a difference in the accuracy of a home pregnancy test. And ovulation can change from month to month. A fertilized egg also can implant in the uterus at different times. That can affect the timing of when HCG starts to be made and when it can be found with a home pregnancy test. Irregular menstrual cycles also can affect pregnancy test results, as they make it hard to figure out when a period should start.

For most home pregnancy tests, you put the end of the test in your urine stream, dip the test in a container of urine or put several drops of urine onto the test. A few minutes later, the result appears. It's often a plus or a minus sign, the words "yes" or "no," one line or two lines, or the words "pregnant" or "not pregnant."

Make sure to follow the test directions for how long to wait before checking the results. It's usually two minutes or more. Most tests also have a control indicator. That's usually a line or another symbol that you can see in the result window. If you don't see that line or symbol, then the test isn't working. Try again with another test.

Some home pregnancy tests are more sensitive than others. More-sensitive tests need less HCG in urine to show a positive result. If you're not sure which type of test to use, ask a pharmacist.

Always check the test's expiration date. If it's past that date, don't use it. Read and follow the test directions carefully.

Many home pregnancy tests claim to be 99% accurate. But home pregnancy tests differ in their ability to find a pregnancy in people who have recently missed a period. If you get a negative test result, but you still think you might be pregnant, take another test one week after your missed period or contact your health care provider.

Fertility medication or other medicine that contains HCG might affect home pregnancy test results. Most medicines, though, including antibiotics and birth control pills, don't affect the accuracy of home pregnancy tests.

That's rare. But it is possible to get a positive result from a home pregnancy test when you're not pregnant. This is called a false-positive.

A false-positive might happen if you had a pregnancy loss soon after the fertilized egg attached to the uterine lining. You also may get a false-positive if you take a pregnancy test soon after taking fertility medicine that contains HCG. Problems with the ovaries and menopause also might lead to a false-positive test result.

It's possible to get a negative result from a home pregnancy test when you are pregnant. This is known as a false-negative. You might get a false-negative if you:

• Take the test too early. The earlier you take a home pregnancy test, the harder it is for the test to find HCG. For the most accurate results, take a home pregnancy test after the first day of a missed period.
• Check test results too soon. Set a timer to go off at the time the test directions say you should check the result. Don't check the test until that amount of time has passed.
• Take the test later in the day. For the most accurate results, take the test right after you get up in the morning. That's when your urine is the most concentrated, making HCG easier to find.

Based on the 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 the pregnancy. The sooner a pregnancy is confirmed, the sooner prenatal care can begin.
• Your home pregnancy test is negative. If your period doesn't begin, take the test again in a few days or in one week. It's especially important to do this if you took the test before or right after a missed period.
• You continue to get negative test results, but your period doesn't start. Or you still think you might be pregnant. Contact your health care provider. Your provider may suggest you take a blood test to check for pregnancy, which may be more accurate than a home test. Also, some health problems may lead to missed periods. If you're not pregnant, your health care provider can help you find out what's causing missed periods.