What is the first-trimester screening for birth defects?
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 or amniocentesis, to find out if there is a problem. Some tests may not be available in all areas. Talk with your doctor about the tests that are right for you.
The first-trimester screening combines the results of two tests.
- Nuchal translucency test . This test uses ultrasound to measure the thickness of the area at the back of the baby's neck. An increase in the thickness can be an early sign of Down syndrome. The test is not available everywhere, because a doctor must have special training to do it.
- First-trimester blood tests. These tests measure the amounts of two substances in your blood: beta human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein A (PAPP-A). Beta-hCG is a hormone made by the placenta. High or low levels may be related to certain birth defects. PAPP-A is a protein in the blood. Low levels may be related to certain birth defects.
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.
How are the tests 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 hand-held 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:
- 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.footnote 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.footnote 1
- The integrated test (first-trimester tests plus the quad screening in the second trimester) correctly finds Down syndrome in 96 out of 100 fetuses who have it. This also means that the test misses Down syndrome in 4 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 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.
Should you have first trimester screening?
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 counsellor. The counsellor 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.
Current as of: May 29, 2019
Author: Healthwise Staff
Sarah Marshall MD - Family Medicine
Thomas M. Bailey MD - Family Medicine
Adam Husney MD - Family Medicine
Kathleen Romito MD - Family Medicine
Siobhan M. Dolan MD, MPH - Reproductive Genetics
Current as of: May 29, 2019