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First-Trimester Screening for Birth Defects

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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.

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.

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WebMD Medical Reference from Healthwise

Last Updated: April 04, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
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