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First-Trimester Screening for Birth Defects - Topic Overview

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, to find out if there is a problem.

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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 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 handheld 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.

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