http://www.cdc.gov/ncbddd/birthdefects/downsyndrome.html
Causes and Risk Factors
The extra chromosome 21 leads to the physical features and developmental challenges that can occur among people with Down syndrome. Researchers know that Down syndrome is caused by an extra chromosome, but no one knows for sure why Down syndrome occurs or how many different factors play a role. One factor that increases the risk for having a baby with Down syndrome is the mother’s age. Women who are 35 years or older when they become pregnant are more likely to have a pregnancy affected by Down syndrome than women who become pregnant at a younger age.3-5However, the majority of babies with Down syndrome are born to mothers less than 35 years old, because there are many more births among younger women.6,7
Diagnosis
Doctors can diagnose Down syndrome during pregnancy or after the baby is born. Some families want to know during pregnancy whether their baby has Down syndrome. Diagnosis of Down syndrome during pregnancy can allow parents and families to prepare for their baby’s special needs.
During pregnancy
There are two basic types of tests available to detect Down syndrome during pregnancy. Screening tests are one type and diagnostic tests are another type. A screening test can tell a woman and her healthcare provider whether her pregnancy has a lower or higher chance of having Down syndrome. So screening tests help decide whether a diagnostic test might be needed. Screening tests do not provide an absolute diagnosis, but they are safer for the mother and the baby. Diagnostic tests can typically detect whether or not a baby will have Down syndrome, but they can be more risky for the mother and baby. Neither screening nor diagnostic tests can predict the full impact of Down syndrome on a baby; no one can predict this.
Screening tests
Screening tests often include a combination of a blood test, which measures the amount of various substances in the mother’s blood (e.g., MS-AFP, Triple Screen, Quad-screen), and an ultrasound, which creates a picture of the baby. During an ultrasound, one of the things the technician looks at is the fluid behind the baby’s neck. Extra fluid in this region could indicate a genetic problem. These screening tests can help determine the baby’s risk of Down syndrome. Rarely, screening tests can give an abnormal result even when there is nothing wrong with the baby. Sometimes, the test results are normal and yet they miss a problem that does exist.
A new test available since 2010 for certain chromosome problems, including Down syndrome, screens the mother’s blood to detect small pieces of the developing baby’s DNA that are circulating in the mother’s blood. This test is recommended for women who are more likely to have a pregnancy affected by Down syndrome. The test is typically completed during the first trimester (first 3 months of pregnancy) and it is becoming more widely available.
Diagnostic Tests
Diagnostic tests are usually performed after a positive screening test in order to confirm a Down syndrome diagnosis. Types of diagnostic tests include:
- Chorionic villus sampling (CVS)—examines material from the placenta
- Amniocentesis—examines the amniotic fluid (the fluid from the sac surrounding the baby)
- Percutaneous umbilical blood sampling (PUBS)—examines blood from the umbilical cord
These tests look for changes in the chromosomes that would indicate a Down syndrome diagnosis.
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