Chromosomes are structures within each of our cells that contain all our genetic information. Normally, each cell contains 23 pairs of chromosomes. Some abnormalities however, can happen after conception. Trisomy 21, commonly known as Down syndrome, is a condition caused by the presence of an extra copy of chromosome 21 (or part of a chromosome) in each cell. Although trisomy is a genetic disorder, it is rarely inherited.
Down syndrome can be diagnosed before or during birth. Prenatal diagnostic testing can be performed to accurately determine whether or not a fetus has trisomy 21. Amniocentesis and umbilical cord blood sampling are among the tests currently used to detect this condition. These tests however, are not without risks as they are associated with a risk of miscarriage.
Since the risk of conceiving a child with Down syndrome increases with the mother's age, these tests are generally recommended for women 35 years and over. For example, if you are 25 years old, your risk of having a child with trisomy 21 is 1/135, whereas if you are 45 years old, your risk is 1/28.
An initial diagnosis can also be made at birth based on the child's physical features. The diagnosis is then confirmed with a blood test.
Given the consequences and risks associated with prenatal diagnostic testing, other screening tests can also be used to determine a child's risk of having trisomy 21. These tests involve ultrasonography and blood tests performed between 15 and 20 weeks. If a pregnancy is considered high risk, an amniocentesis can then be recommended to confirm whether or not the fetus has Down syndrome.
The physical features of persons with trisomy 21 are quite distinct and easy to recognize:
About 40% of babies with Down syndrome are born with a heart defect, and 10% are born with a bowel obstruction. Children with trisomy 21 are also more likely to have recurrent otitis media (middle ear infections) which may result in hearing loss and language delays. Hypothyroidism, leukemia, seizure disorders and vision problems are also associated with this population. Furthermore, mild to moderate mental retardation affects their ability to learn.
Those affected may experience premature aging, and Alzheimer's disease is also more prevalent in Down syndrome sufferers than in the general population.
There is no treatment or cure for trisomy 21. We can however, improve the quality of life of those affected, and help them become more independent.
With constant supervision and stimulation starting at a very early age, children with Down syndrome can now be sent to school.
Thanks to advances in therapy, 80% of adults with Down syndrome can now expect to live beyond 55 years.
For more information or for support:
Canadian Down Syndrome Society
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