First-Trimester Screening for Birth Defects - Topic Overview
How are the tests done? continued...
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:
- Wrap an elastic band around your upper arm. This makes the veins below the band larger so it is easier to put a needle into the vein.
- Clean the needle site with alcohol.
- Put the needle into the vein.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Put a gauze pad or cotton ball over the needle site as the needle is removed.
- Put pressure on the site and then put on a bandage.
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.
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.