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The triple screening measures
the amounts of three substances in a pregnant woman's blood: alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and estriol (uE3). When a test for the hormone inhibin A is added, it's called a
quad screening. These tests are also called the maternal serum triple or quad test, the
expanded AFP test, the AFP plus test, or the multiple marker screening test.
The amounts of these substances help your doctor find out the chance that
your baby has certain birth defects, such as
spina bifida, or
anencephaly. These tests can't show for sure that your baby has a birth defect. You would need a diagnostic test called amniocentesis to find out for sure if there is a problem.
The triple or quad screening is usually done at 15
to 20 weeks of pregnancy.
Each substance tested in a triple or
quad screening gives you different information about possible problems.
Together, these results give the best information. These screening tests look for the amount of:
Your doctor will look at the levels of these substances—along with your age and other factors—to see if your baby has a higher-than-average chance of having a birth defect.
A simple blood test is all that's needed for these tests.
The health professional taking a sample of your blood will:
A screening test shows the chance that a baby has a certain birth defect. The accuracy of a screening test is based on how often the test correctly finds a birth defect.
With the triple or quad test, there is a chance of getting a
false-positive test result. This means that the test
could show a problem when the baby doesn't have the problem. A false positive may be more likely with the triple screening than the quad screening.
A false-positive result can cause stress and lead to unnecessary testing (such as
an amniocentesis). Many women who have a positive screening test result are actually carrying a healthy baby.
Sometimes negative test results can be wrong too. They may show that the baby is fine when he or she does have a birth defect. (This is a false-negative test result.)
Your doctor will use your age and your baby's age to interpret the test results. If your test results are
abnormal, your doctor may use a
fetal ultrasound to make sure of your baby's age.
A "positive" result means that there is a higher-than-average chance your baby has a birth defect. If
the result is "negative," or normal, it means that your baby probably
doesn't have a birth defect. But it doesn't guarantee that you will have a
normal pregnancy or baby.
Your doctor may tell you the result of your test as a set of numbers. Doctors often use a certain number as a cutoff for a positive result. For example, your doctor may say the cutoff is 1 out of 200. This means that if your result is 1 out of 200 or 1 out of a number less than 200 (such as 1 out of 100), you have a positive result and your baby has a higher chance of a birth defect. If your result is 1 out of 300, this means that you have a negative result and your baby has a lower chance of a birth defect.
If you have a positive test result, your doctor may want you to have the diagnostic test amniocentesis to find out for sure if there is a
problem. But it's your choice whether to have another test.
If you have a negative result, you may choose not to have another test.
Deciding whether to have a test for birth defects is a personal decision. And it can be a hard choice. You need to think about what the results of a test would mean to you and how they might affect your choices about your pregnancy.
If you choose to have a test, you may want to
talk with a
genetic counselor. The counselor can talk with you
about the reasons to have or not have the test. He or she can also help you
find other resources for support and decision-making.
For more help on deciding about having a triple or quad screening, see the topic:
Cunningham FG, et al. (2010). Prenatal diagnosis and
fetal therapy. In Williams Obstetrics, 23rd ed., pp.
287–311. New York: McGraw-Hill Medical.
American College of Obstetricians and Gynecologists
(2007, reaffirmed 2008). Screening for fetal chromosomal abnormalities. ACOG Practice Bulletin
No. 77. Obstetrics and Gynecology, 109(1): 217–227.
April 4, 2012
Sarah Marshall, MD - Family Medicine & Siobhan M. Dolan, MD, MPH - Reproductive Genetics
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