Screening tests for birth defects done in the first trimester of pregnancy can be done using a sample of your blood and using ultrasound to measure the thickness of the skin at the back of your baby's neck.

Blood tests

A health professional uses a needle to take a blood sample, usually from the arm.

Nuchal translucency ultrasound test

A small, hand-held device is moved over your belly to display images of the baby. The thickness of the fluid buildup at the back of the baby's neck is measured.

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First-trimester screening shows the chance of your baby having certain birth defects such as Down syndrome or trisomy 18.

Your doctor may tell you the result of your test as a set of numbers.

• A positive result means that there is a higher-than-average chance that your baby has Down syndrome or trisomy 18. 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. This means that your baby has a higher chance of a birth defect.
• A negative result means that your baby probably doesn't have those birth defects. 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. But it doesn't guarantee that you will have a normal pregnancy or baby.

If you have a positive test result, your doctor may give you the option to have 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.

Accuracy of the test

First-trimester screening (nuchal translucency combined with 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.^{footnote 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 testing you don't need (such as chorionic villus sampling [CVS]). Many women who have a positive screening test result are actually carrying a healthy baby.