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Genetic Testing

What is Down syndrome?

Down syndrome is one of many chromosome disorders. Chromosomes are the genetic blueprints that determine how a baby will develop. Chromosome disorders occur when a baby receives too few or too many chromosomes at conception. Down syndrome occurs when a baby receives an extra chromosome 21. This disrupts development and causes birth defects and mental retardation. These disorders are not typically inherited. Instead, they are usually caused by an error during formation of the egg or sperm or during the earliest stages of development.

What is fetal genetic testing?

Screening tests are blood and/or ultrasound tests that estimate the likelihood for the fetus to have certain types of birth defects, including Down syndrome, Trisomy 18, Trisomy 13, sex chromosome abnormalities (that is, Turner, Klinefelter, Jacobs, and Triple X syndromes), and certain microdeletion syndromes. A normal result does not guarantee a normal/healthy baby, as these tests are not diagnostic tests and only screen for certain conditions.

Who should have fetal genetic screening?

Screening is offered to all pregnant women who are at least 10 weeks pregnant. Because women over age 34 or with a family history of chromosome abnormalities are at a higher risk of having a baby with chromosome abnormalities, the American College of Obstetricians and Gynecologists recommends that all be offered chorionic villi sampling or an amniocentesis for chromosome testing. These tests detect 99.9% of all chromosome abnormalities. However, these diagnostic tests are invasive and carry a small risk of miscarriage.

How and when is genetic screening done?

There are several ways to do genetic testing/screening. If you choose to undergo genetic screening, you will only need to have one of these tests performed:

First-trimester ultra screen: This test is done between 12 and 13 6/7 weeks and consists of 1) a special sonogram to measure the nuchal translucency (NT) – the amount of fluid behind the neck of the baby and 2) blood testing to measure protein levels (B-HCG and PAPP-A). This test detects more than 90% of Down syndrome, Trisomy 18, and Trisomy 13. This test does not detect spinal defects. Therefore, further testing with a sonogram will be done at 18-20 weeks to detect spinal defects.

Triple Screen: This test is done between 16 and 18 weeks of pregnancy. It measures three different chemicals in the blood. No sonogram is necessary. It will detect 60 to 70% of babies with Down syndrome and 80% of babies with spinal defects.

Am I required to have genetic testing?

NO. The decision whether to have the screening tests performed is your personal decision. If these tests show that your baby is at an increased risk for a chromosome abnormality, it does not mean a problem has been diagnosed, only that further evaluation, usually with an amniocentesis, is indicated. This can cause a lot of anxiety as well as some extra expense. Therefore, the decision to have genetic screening is left to the patient, rather than simply testing all patients.

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