How do pregnancy tests work?
All pregnancy tests look for a special hormone in the urine or blood that is
only present when a woman is pregnant. This hormone, human chorionic
gonadotropin (hCG), is also called the pregnancy hormone.
What are the types of pregnancy tests?
There are two types of pregnancy tests - blood and urine tests.
Both tests
look for the presence of hCG, the pregnancy hormone. Today, many women use a
urine test, or home pregnancy test (HPT), to find out if they are pregnant. HPTs
do not cost a lot, are easy to use, can be done at home, and are private.
When a
woman has a positive result on an HPT, she needs to see her health care provider
right away. The health care provider can confirm a positive HPT result with a
blood test and a pelvic exam.
There are two types of blood tests you can get from a health care provider. A
quantitative blood test (or the beta hCG test) measures the
exact amount of hCG in the blood. This means it can pick up very small amounts
of hCG, making it a very accurate test. A qualitative hCG blood test
gives a simple yes or no answer to whether you are pregnant. This test is more
like a urine test in terms of its accuracy.
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).
Urine tests can determine pregnancy about 2 weeks after ovulation. Some more
sensitive urine tests can tell if you are pregnant as early as 6 days after you
conceive, or one day after you miss a menstrual period.
How is a home pregnancy test done?
There are many types of home pregnancy tests, or HPTs, that can be bought
over-the-counter at drug or discount stores. Some involve collecting your urine
in a cup and dipping a stick into the urine, or putting some of the urine into a
special container with an eyedropper. Others are done by placing a stick into
your urine stream. Tests vary in how long you have to wait for the stick or
container to turn a certain color or have a symbol on it (like a plus or a
minus). All tests come with written instructions. Most tests also have toll-free
phone numbers to call if you have any questions about how to do the test or read
the results.
How accurate are home pregnancy tests?
Home pregnancy tests (HPTs) are very accurate. Most brands of HPTs say they
are 97% to 99% accurate, but this can vary with actual use. Each brand varies in
how sensitive it is in picking up the pregnancy hormone hCG. If a test is not
done correctly, it will be less accurate. And, always check the package to make
sure it is not past its expiration date - if it is, it will not be accurate.
Most brands of HPTs tell users to do the test again in a few days, no matter
what the results.
If you use an HPT too early in your pregnancy, you may not have enough of the
pregnancy hormone hCG in your urine to have a positive test result. Most HPTs
will be accurate if you test yourself around the time your period is due (about
2 weeks after you ovulate, or release an egg from your ovary). You can get a
negative test result if you are not pregnant or if you ovulated later than you
thought you did. You may also have problems with the pregnancy, which affects
the amount of hCG you have in your urine. If your HPT is negative, test yourself
again within a few days to 1 week. If you keep getting a negative result and
think you are pregnant, talk with a health care provider right away.
Can anything interfere with home pregnancy test results?
Most medications, both over-the-counter and prescription drugs, including
birth control pills and antibiotics, should not affect the results of a home
pregnancy test (such as Profasi, Pregnyl or Novarel). Only those drugs that have
the pregnancy hormone hCG in them can give a false positive test result
(where the test says you are pregnant when you actually are not). Drugs that
have hCG in them can be used for treating infertility (not being able to get
pregnant).
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