Is your period late? Are you nauseated or fatigued? Are your breasts tender? It might be time to consider a home pregnancy test.

Regardless of the circumstances, home pregnancy tests can be nerve-racking. To quell your anxiety, find out how home pregnancy tests work, what can affect the results and when to visit the doctor.

How do home pregnancy tests detect pregnancy?

Shortly after the fertilized egg attaches to your uterine lining, your body begins to produce the hormone human chorionic gonadotropin (HCG). Home pregnancy tests — available without a prescription at most grocery stores and drugstores — can reliably detect this hormone in your urine one week after a missed period. A more sensitive blood test to detect HCG can be done earlier in the doctor's office.

Are there different types of home pregnancy tests?

Various types of home pregnancy tests are available, but most work in a similar way. Typically, you'll place the end of a dipstick in your urine stream or immerse the dipstick in a container of collected urine for five to ten seconds. A minute or two later, you'll see a plus or minus sign, a line, a color change or the words "pregnant" or "not pregnant" on a strip or window on the dipstick.

With some tests, you'll mix a small amount of collected urine with a special liquid or powder. If the urine changes color, the test is positive.

Instructions may vary slightly from kit to kit. Read the instructions carefully before you take the test. If you have questions about how to do the test or interpret the results, call the manufacturer. Look for a toll-free number in the package instructions.

When should I take a home pregnancy test?

Many home pregnancy tests can be taken as early as the first day of a missed period. For the most accurate results, it's best to wait until one week after a missed period.

How accurate are the results?

Many home pregnancy tests claim to be 99 percent accurate on the day you miss your period. Although research suggests that most home pregnancy tests don't consistently spot pregnancy this early, the tests are considered reliable when used according to package instructions one week after a missed period. If you need to know earlier, ask your doctor about a blood test.

Could a positive result be wrong?

Rarely, 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.

You may get a false-positive if you have traces of blood or protein in your urine. Various prescription drugs — including diuretics and promethazine (used to treat allergy symptoms and nausea) — also can cause a false-positive result. Using an expired or damaged test kit may have the same effect.

Could a negative result be wrong?

More commonly, you can get a negative result from a home pregnancy test when you're actually pregnant. This is known as a false-negative. You may get a false-negative if you:

Take the test too early. Wait to take the test until your period is at least one day late.That's the earliest most home pregnancy tests can detect pregnancy. For the most accurate results, take the test one week after a missed period.

Time the test wrong. If you're collecting urine, do the test within 15 minutes of collecting the sample. And be sure to give the test time to work — but not too much time. You may want to set a timer according to the package instructions.

Use diluted urine. Drinking too much fluid before taking the test may 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.

If your period hasn't started within a week after a negative home pregnancy test, repeat the test.

What happens next?

If your home pregnancy test is positive — or if you've taken a few home pregnancy tests and gotten mixed results — make an appointment with your doctor, nurse practitioner or midwife. You may need a blood test or pelvic exam to confirm your pregnancy. The sooner your pregnancy is confirmed, the sooner you can begin prenatal care.

If your home pregnancy test is still negative two weeks after a missed period, check with your doctor. Many things can cause missed periods, including illness, stress, excessive exercise and hormonal imbalances. Your doctor can help you get your menstrual cycle back on track.