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Pregnancy Tests

Pregnancy Tests How do pregnancy tests work?
Pregnancy tests look for a special hormone in the urine or blood that is only there when a woman is pregnant. This hormone, human chorionic gonadotropin (hCG), can also be called the pregnancy hormone.

The pregnancy hormone, hCG, is made in your body when a fertilized egg implants in the uterus. This usually happens about 6 days after conception. But studies show that the embryo doesn't implant until later in some women. The amount of hCG increases drastically with each passing day you are pregnant.

Many home pregnancy tests claim they can tell if you're pregnant on the day you expect your period. But a recent study shows that most don't give accurate results this early in pregnancy. Waiting one week after a missed period will usually give a more accurate answer.

What's the difference between pregnancy tests that check urine and those that test blood? Which one is better?
There are two types of pregnancy tests. One tests the blood for the pregnancy hormone, hCG. The other checks the urine for this hormone. You can do a urine test at home with a home pregnancy test. You need to see a doctor to have blood tests.

These days, most women first use home pregnancy tests (HPT) to find out if they are pregnant. HPTs are inexpensive, private, and easy to use. Urine tests will be able to tell if you're pregnant about 2 weeks after ovulation. Some more sensitive urine tests claim that they can tell if you are pregnant as early as one day after a missed period.

If a HPT says you are pregnant, you should call your doctor right away. You 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.

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 6 to 8 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. Qualitative hCG blood tests just check to see if the pregnancy hormone is present or not. So it gives a yes or no answer. The qualitative hCG blood test is about as accurate as a urine test.

How do you do a home pregnancy test?
There are many different types of home pregnancy tests, or HPTs. Most drugstores sell HPTs over-the-counter. They cost between $8 and $20 depending on the brand and how many tests come in the box.

Most popular HPTs work in a similar way. The majority tell 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 put a few drops into a special container with a dropper. Testing the urine first thing in the morning may help boost accuracy.

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.

Most tests also have a "control indicator" in the result window. This line or symbol shows whether the test is working or not. 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 incase of questions about use or results.

How accurate are home pregnancy tests?
Home pregnancy tests (HPTs) can be quite accurate. But the accuracy depends on many things. These include:

How you use them
- Be sure to follow the directions and check the expiration date.

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. If you wait one week after a missed period to test, you are more apt to have an accurate result. Also, testing your urine first thing in the morning may boost the accuracy.

Who uses them
- The amount of hCG in the urine is different for every pregnant woman. So, some women will have accurate results on the day of the missed period while others will need to wait longer.

The brand of test
- Some home pregnancy tests are more sensitive than others. So, some tests are better than others at spotting hCG early on.

Many HPTs claim to be 99% accurate on the day you miss your period. But research suggests that most HPTs do not consistently spot pregnancy that early. And when they do, the results are often very faint.

In a 2004 study, researchers tested the accuracy of 18 HPTs sold in retail stores. They found that only one brand consistently detected the low levels of hCG usually present on the first day of the missed period. This was the First Response, Early Result Pregnancy Test. The other tests missed up to 85% of pregnancies on the first day of the missed period. Most tests accurately confirmed pregnancies one week after the missed period.

Which brand of pregnancy test is the most accurate?
Some brands of tests can pick up lower levels of hCG than others. But limited research makes it impossible to say for sure which one is the best. Even so, two studies suggest that First Response, Early Result Pregnancy Test may be more sensitive than others. So for women who want test early, this is probably the best product.

How soon after a missed period can I take a home pregnancy test and get accurate results?
Many home pregnancy tests (HPTs) claim to be 99% accurate on the day you miss your period. But research suggests that most HPTs do not consistently 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 home pregnancy tests will give you an accurate answer. Ask your doctor for a more sensitive test if you need to know earlier.

When a home pregnancy test will give an accurate result depends on many things. These include:

How long it takes for the fertilized egg to implant in the uterus after ovulation?
Pregnancy tests look for the hormone human chorionic gonadotropin (hCG) that is only produced once the fertilized egg has implanted in the uterine wall. In most cases, this happens about 6 days after conception. But studies show that in up to 10 percent of women, the embryo doesn't implant until much later, after the first day of the missed period. So, home pregnancy tests will be accurate as soon as one day after a missed period for some women but not for others.

How you use them
- Be sure to follow the directions and check the expiration date.

When you use them
- The amount of hCG in a pregnant woman's urine increases with time. So, the earlier after a missed period you take a HPT, the harder it is to spot the hCG. If you wait one week after a missed period to test, you are more apt to have an accurate result. Also, testing your urine first thing in the morning may boost the accuracy.

Who uses them
- The amount of hCG in the urine at different points in early pregnancy is different for every woman. So, some women will have accurate results on the day of the missed period while others will need to wait longer.

The brand of test
- Some home pregnancy tests are more sensitive than others. So, some tests are better than others at spotting hCG early on.

I got a negative result on a home pregnancy test. Might I still be pregnant?
Yes. So, most HPTs suggest women take the test again in a few days or a week.

Every woman ovulates at different times in her menstrual cycle. Plus, embryos implant in the 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 the test says you are not pregnant and you are) when they test too early in the pregnancy. 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 1 week. If you keep getting a negative result but think you are pregnant, talk with your doctor right away.

Can anything interfere with home pregnancy test results?
Most medicines, over-the-counter and prescription, including birth control pills and antibiotics, should not affect the results of a home pregnancy test. 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 women who may become pregnant should not use these substances.

For more information on pregnancy tests, contact the National Women's Health Information Center at 1-800-994-9662 or contact the following organizations:

Food and Drug Administration
Phone Number(s): (888) 463-6332
Internet Address: http://www.fda.gov

American College of Obstetricians and Gynecologists
Phone Number(s): (800) 762-2264 for publication requests only
Internet Address: http://www.acog.org/

Planned Parenthood Federation of America
Phone Number(s): (800) 230-7526
Internet Address: http://www.plannedparenthood.org

American College of Nurse-Midwives
Phone Number(s): (888) 643-9433
Internet Address: http://www.midwife.org

This FAQ was reviewed by:
Kerri Parks, MD
Assistant Professor
Los Angeles County Women's and Children's Hospital
USC-Keck School of Medicine
Los Angeles, CA

Mory Nouriani, MD
Sher Institute of Reproductive Medicine
Glendale, CA



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