Near the end of the first 3 months of pregnancy (first trimester), a woman can have two types of tests to show the chance that her baby has a birth defect. When the results are combined, these tests are known as the first-trimester screening. They also may be called the combined first-trimester screening or the combined screening.
These screening tests help your doctor find out the chance that
your baby has certain birth defects, such as
Down syndrome or
trisomy 18. These tests can't show for sure that your baby has a birth defect. You would need a diagnostic test, such as a chorionic villus sampling, to find out if there is a problem.
The first-trimester screening combines the results of two tests.
First-trimester screening also may be done as part of an integrated screening test. This combines the results of the first-trimester tests with those of second-trimester screening (a blood test called the triple or quad screening). You would get the results after the second-trimester test is done.
For the nuchal translucency test, your doctor or an ultrasound technologist spreads a gel on your belly. Then he or she gently moves a handheld device called a transducer over your belly. Images of the baby are displayed on a monitor. The doctor can look for and measure the thickness at the back of the baby's neck.
A simple blood test is all that's needed for the rest of the first-trimester screening.
The health professional taking a sample of your blood will:
There are no known physical risks to having the tests, other than a possible bruise on your arm from the blood test.
The doctor looks at the test results—along with your age and other factors—to find out the chance
that your baby may have certain problems.
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.
It's possible that a screening test will be positive—meaning the test result is abnormal—but the baby doesn't have the problem. This is called a false-positive test result. And it's also possible that a screening may show that a baby doesn't have a birth defect when he or she does have it. This is called a false-negative test result.
A false-positive result can cause stress and lead to unnecessary testing (such as chorionic villus sampling [CVS]). Many women who have a positive screening test result are actually carrying a healthy baby.
A "positive" result means that there is a higher-than-average chance your baby has Down syndrome or trisomy 18. If
the result is "negative," it means that your baby probably
doesn't have those birth defects. 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 chorionic villus sampling (CVS) or an amniocentesis test in the second trimester to find out if your baby has a problem. But it's your choice whether to have another test.
If you have a negative result, you may choose not to have any more tests.
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 an ultrasound, see:
American College of Obstetricians and Gynecologists (2007, reaffirmed 2011). 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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