What is Down Syndrome?
Down Syndrome (DS) is one of many chromosome disorders. Chromosomes are the genetic blueprints that determine how a baby will develop. Chromosome disorders occur by chance when a baby receives too few or too many chromosomes at conception. DS occurs when a baby receives an extra chromosome 21. This disrupts development and causes birth defects and mental retardation.
What is genetic screening?
Screening tests are blood and or ultrasound tests that estimate the likelihood for the fetus to have certain types of birth defects, including DS, Trisomy 13 and Trisomy 18. A normal result does not necessarily mean that no abnormalities are present. They do not guarantee a normal baby. Screening tests always have a certain amount of false positives (an abnormal test in a baby with no birth defects) and false negatives (a normal test in a baby that has birth defects).
Who should have genetic screening?
Screening is offered to all pregnant women who are between 11 – 18 weeks pregnant. Because they are at a higher risk of having a baby with chromosome abnormalities, the American College of Obstetricians and Gynecologists recommends that all women over the age of 34 or with a family history of chromosome abnormalities be offered chorionic villi sampling or an amniocentesis for chromosome testing. These tests detect 99.9% of all chromosome abnormalities. In contrast, screening detects 60 – 95% of DS, Trisomy 18 and Trisomy 13. Five to 10% of babies with these chromosome problems will not be detected with screening alone.
How and when is genetic screening done?
There are two ways to do genetic testing. If you choose to undergo genetic screening, you will only need to have one of these tests performed.
- 1st trimester screening: This test is done between 11 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 (free B-HCG and PAPP-A). This test detects >90% of Down Syndrome, Trisomy 18 & Trisomy 13. This test does not detect spinal defects like spina bifida or abdominal wall defects. Therefore, further testing with a sonogram will be done at 18 – 20 weeks to detect these defects.
- 2nd trimester screening (Quad screen): This test is done between 16 – 18 weeks of pregnancy. It measures 4 different chemicals in the maternal blood. No sonogram is necessary. It will detect 60 – 70% of babies with Down Syndrome and 80% of babies with spinal and abdominal wall 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. These are reasons why the decision to have genetic screening is left to the patient rather than just doing it on all patients.