*Pregnancy test facts medically edited by: Charles Patrick Davis, MD, PhD

• Pregnancy tests work by detecting a hormone in a woman's blood or urine: hCG (human chorionic gonadotropin) is produced when an egg implanted in the uterus.
• There are two types of pregnancy tests, one uses blood and the other uses urine (home tests); a quantitative blood test detects hCG earlier than a qualitative blood or urine test.
• Most home pregnancy tests detect hCG in a urine stream when, after a few minutes, a line or symbol appears if the test is positive (indicating pregnancy); most urine pregnancy test instructions urge a second test in a few days no matter what were the first results; follow package instructions – if a control indicator does not appear, the test result should not be trusted.
• Home tests are accurate, some better (more sensitive in showing early pregnancy) than others.
• Home pregnancy tests may test positive on the first day of a missed period, but almost all detect pregnancy 1 week after a missed period.
• Call your doctor for an appointment if you test positive for pregnancy with a home test.
• It is possible to be pregnant and have a negative home pregnancy test; that is why you should wait about a week to repeat the test. Most medicines and other compounds (alcohol, drugs) do not interfere with pregnancy tests; however, any drug containing hCG (some fertility drugs) may give false test results.
• Any questions or problems with pregnancy tests should be discussed with your doctor.

All pregnancy tests work by detecting a certain hormone in the urine or blood that is only there when a woman is pregnant. This hormone is called human chorionic gonadotropin (kohr-ee-ON-ihk goh-NAD-uh-TROH-puhn), or hCG. It is also called the pregnancy hormone.

hCG is made when a fertilized egg implants in the uterus. This usually happens about six days after the egg and sperm merge. But studies show that in up to 10 percent of women, implantation does not occur until much later, after the first day of the missed period. The amount of hCG rapidly builds up in your body with each passing day you are pregnant.

Yes. There are two types of pregnancy tests. One tests the blood for the pregnancy hormone, hCG. You need to see a doctor to have a blood test. The other checks the urine for the hCG hormone. You can do a urine test at a doctor's office or at home with a home pregnancy test (HPT).

These days, many women first use an HPT to find out if they are pregnant. HPTs are inexpensive, private, and easy to use. HPTs also are highly accurate if used correctly and at the right time. HPTs will be able to tell if you're pregnant about one week after a missed period.

Doctors use two types of blood tests to check for pregnancy. Blood tests can pick up hCG earlier in a pregnancy than urine tests can. Blood tests can tell if you are pregnant about six to eight days after you ovulate (or release an egg from an ovary). A quantitative blood test (or the beta hCG test) measures the exact amount of hCG in your blood. So it can find even tiny amounts of hCG. This makes it very accurate. A qualitative hCG blood test just checks to see if the pregnancy hormone is present or not. So this test gives a yes or no answer. The qualitative hCG blood test is about as accurate as a urine test.

There are many different types of home pregnancy tests (HPTs). Most drugstores sell HPTs over the counter. They are inexpensive. But the cost depends on the brand and how many tests come in the box.

Most HPTs work in a similar way. Many instruct the user to hold a stick in the urine stream. Others involve collecting urine in a cup and then dipping the stick into it. At least one brand tells the woman to collect urine in a cup and then use a dropper to put a few drops of the urine into a special container. Then the woman needs to wait a few minutes. Different brands instruct the woman to wait different amounts of time. Once the time has passed, the user should inspect the "result window." If a line or plus symbol appears, you are pregnant. It does not matter how faint the line is. A line, whether bold or faint, means the result is positive. New digital tests show the words "pregnant" or "not pregnant."

Most tests also have a "control indicator" in the result window. This line or symbol shows whether the test is working properly. If the control indicator does not appear, the test is not working properly. You should not rely on any results from a HPT that may be faulty.

Most brands tell users to repeat the test in a few days, no matter what the results. One negative result (especially soon after a missed period) does not always mean you're not pregnant. All HPTs come with written instructions. Most tests also have toll-free phone numbers to call in case of questions about use or results.

Home pregnancy tests (HPTs) can be quite accurate. But the accuracy depends on:

• How you use them: Be sure to check the expiration date and follow the instructions. Wait ten minutes after taking the test to check the results window. Research suggests that waiting 10 minutes will give the most accurate result.
• When you use them: The amount of hCG or pregnancy hormone in your urine increases with time. So, the earlier after a missed period you take the test, the harder it is to spot the hCG. Many HPTs claim to be 99 percent accurate on the first day of your missed period. But research suggests that most HPTs do not always detect the low levels of hCG usually present this early in pregnancy. And when they do, the results are often very faint. Most HPTs can accurately detect pregnancy one week after a missed period. Also, testing your urine first thing in the morning may boost the accuracy.
• Who uses them: Each woman ovulates at a different time in her menstrual cycle. Plus, the fertilized egg can implant in a woman's uterus at different times. hCG only is produced once implantation occurs. In up to 10 percent of women, implantation does not occur until after the first day of a missed period. So, HPTs will be accurate as soon as one day after a missed period for some women but not for others.
• The brand of test: Some HPTs are more sensitive than others. So, some tests are better than others at spotting hCG early on.

Many home pregnancy tests (HPTs) claim to be 99 percent accurate on the first day of your missed period. But research suggests that most HPTs do not always spot pregnancy that early. And when they do, the results are often so faint they are misunderstood. If you can wait one week after your missed period, most HPTs will give you an accurate answer. Ask your doctor for a more sensitive test if you need to know earlier.

If a home pregnancy test is positive and shows that you are pregnant, you should call your doctor right away. Your doctor can use a more sensitive test along with a pelvic exam to tell for sure if you're pregnant. Seeing your doctor early on in your pregnancy will help you and your baby stay healthy.

Yes. So, most home pregnancy tests (HPTs) suggest women take the test again in a few days or a week if the result is negative.

Each woman ovulates at a different time in her menstrual cycle. Plus, the fertilized egg can implant in a woman's uterus at different times. So, the accuracy of HPT results varies from woman to woman. Other things can also affect the accuracy. Sometimes women get false negative results when they test too early in the pregnancy. This means that the test says you are not pregnant when you are. Other times, problems with the pregnancy can affect the amount of hCG in the urine.

If your HPT is negative, test yourself again in a few days or one week. If you keep getting a negative result but think you are pregnant, talk with your doctor right away.

Most medicines should not affect the results of a home pregnancy test (HPT). This includes over-the-counter and prescription medicines, including birth control pills and antibiotics. Only medicines that have the pregnancy hormone hCG in them can give a false positive test result. A false positive is when a test says you are pregnant when you're not. Sometimes medicines containing hCG are used to treat infertility (not being able to get pregnant).

Alcohol and illegal drugs do not affect HPT results. But do not use these substances if you are trying to become pregnant or are sexually active and could become pregnant.