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During the first trimester of pregnancy, screening tests for birth defects may be done. These tests look for possible problems with your baby. The tests may be called first-trimester screening, combined first-trimester screening, or the combined screening.
The screening tests show the chance of your baby having certain birth defects such as Down syndrome or trisomy 18.
First-trimester screening tests include:
These measure the level of two substances in the blood called pregnancy-associated plasma protein A (PAPP-A) and beta-human chorionic gonadotropin hormone (beta-hCG). PAPP-A is a protein in the blood. Beta-hCG is a hormone made by the placenta. In some areas, other blood tests may be used.
In some cases, the doctors look at the combined screenings that you've had over a period of time. This is called an integrated screening. It's done in two stages at two different times during the pregnancy.
First-Trimester Screening for Birth Defects
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.
A health professional uses a needle to take a blood sample, usually from the arm.
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.
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.
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.
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.
CitationsAmerican College of Obstetricians and Gynecologists (2016). Screening for fetal aneuploidy. ACOG Practice Bulletin No. 163. Obstetrics and Gynecology, 127(5): e123–e137. DOI: 10.1097/AOG.0000000000001406. Accessed April 6, 2017.
Adaptation Date: 6/12/2023
Adapted By: Alberta Health Services
Adaptation Reviewed By: Alberta Health Services
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