First-trimester screening

During the first trimester of pregnancy, screening tests for birth defects and genetic conditions 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 or genetic conditions, such as Down syndrome or trisomy 18. But these tests can't tell you for sure that your baby has a problem.

First-trimester screening tests include:

• Blood tests. 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.
• Nuchal translucency test. This test uses ultrasound to measure the thickness of the skin at the back of your baby's neck. An increase in the thickness can be an early sign of certain genetic conditions, such as Down syndrome.

In some cases, your doctor or midwife looks 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.

Screening tests for birth defects and genetic conditions 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, handheld 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 (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.

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 or genetic condition when they do 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]). In many cases, those with a positive screening test result are actually carrying a healthy baby.