HOW DO PREGNANCY TESTS WORK?
All pregnancy tests look for a special hormone in the urine or blood that is only present in pregnant women. This hormone, human chorionic gonadotropin (hCG), is also called the pregnancy hormone.
WHAT IS THE DIFFERENCE BETWEEN A URINE AND A BLOOD PREGNANCY TEST? IS ONE BETTER THAN THE OTHER?
There are two types of pregnancy tests.
Both tests look for the presence of hCG, the pregnancy hormone. Many women use a urine test or home pregnancy test (HPT) to find out if they are pregnant. HPTs are cheap, 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 caregiver right away. The caregiver can confirm a positive HPT result with another urine test, a blood test, ultrasound, and a pelvic exam.
There are two types of blood tests you can get from a caregiver.
A quantitative blood test (or the beta hCG test). This 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. This 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 release an egg from an ovary (ovulate). 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 or even 1 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 need 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?
HPTs are very accurate. Most brands of HPTs say they are 97% to 99% accurate when taken 1 week after missing your menstrual period, 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. 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). 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 your caregiver right away about getting a blood pregnancy test.
FALSE POSITIVE PREGNANCY TEST
A false positive HPT can happen if there is blood or protein present in your urine. A false positive can also happen if you were recently pregnant or if you take a pregnancy test too soon after taking fertility drug that contains hCG. Also, some prescription medicines such as water pills (diuretics), tranquilizers, seizure medicines, psychiatric medicines, and allergy and nausea medicines (promethazine) give false positive readings.
FALSE NEGATIVE PREGNANCY TEST
A false negative HPT can happen if you do the test too early. Try to wait until you are at least 1 day late for your menstrual period.
It may happen if you wait too long to test the urine (longer than 15 minutes).
It may also happen if the urine is too diluted because you drank a lot of fluids before getting the urine sample. It is best to test the first morning urine after you get out of bed.
If your menstrual period did not start after a week of a negative HPT, repeat the pregnancy test.
CAN ANYTHING INTERFERE WITH HOME PREGNANCY TEST RESULTS?
Most medicines, both over-the-counter and prescription drugs, including birth control pills and antibiotics, should not affect the results of a HPT. Only those drugs that have the pregnancy hormone hCG in them can give a false positive test result. Drugs that have hCG in them may be used for treating infertility (not being able to get pregnant). Alcohol and illegal drugs do not affect HPT results, but you should not be using these substances if you are trying to get pregnant. If you have a positive pregnancy test, call your caregiver to make an appointment to begin prenatal care.