First-Trimester Screening for Birth Defects - Topic Overview
How accurate are the tests?
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.
- The nuchal translucency test correctly finds Down syndrome in 64 to 70 out of 100 fetuses who have it. It misses Down syndrome in 30 to 36 out of 100 fetuses.1
- First-trimester screening (nuchal translucency combined with the blood tests) correctly finds Down syndrome in 82 to 87 out of 100 fetuses who have it. This also means that these tests miss it in 13 to 18 out of 100 fetuses.1
- The integrated
test (first-trimester tests plus the quad screening in the second trimester) correctly finds Down syndrome in about 95 out of 100 fetuses who have it. This also means that the test misses Down syndrome in 5 out of 100 fetuses.1
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.
What do the results mean?
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.
What do you do with the results?
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.