Down Syndrome

What is Down syndrome?
What causes Down syndrome?
Is it possible to test for Down syndrome during pregnancy?
Where can I get more information about Down syndrome?

What is Down syndrome?
Down syndrome is a common chromosome abnormality found in children after birth. Down syndrome affects approximately 1 in every 800 to 1000 newborns.^1,3 A syndrome is a pattern or combination of physical, mental, and health characteristics that are usually seen together. Typically, Down syndrome is recognizable at birth, due to the presence of particular physical features, such as almond shaped eyes, small ears, flattened facial profile, and reduced muscle tone.² Approximately 30% to 50% of newborns with Down syndrome will have some form of heart defect and about 10% will have some type of digestive system defect.³ Many of these birth defects are repairable through surgery with good outcome. Children with Down syndrome will also have some degree of mental retardation and developmental delay. The degree of delay is variable but most commonly in the mild to moderate range.³ This means that most children with Down syndrome will learn to walk and talk, just at a slower pace. People with Down syndrome attend school with help, but it is not likely that they will be able to live independently. There is a wide range of possibilities with Down syndrome, and one cannot predict the degree of physical problems or mental retardation either during pregnancy or early in childhood.

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What causes Down syndrome?
Down syndrome is caused by the presence of an extra copy of genes found on chromosome 21 in the cells of the body. Typically, individuals have 46 chromosomes (23 pairs) in each cell with two copies of chromosome number 1, two copies of chromosome number 2, etc. The 23rd pair determines sex so that males usually have one X and one Y chromosome and females usually have two X chromosomes. About 95% of people with Down syndrome have an extra chromosome 21, which means they have three copies of this chromosome in each cell. This is why Down syndrome is also called trisomy 21.¹ Trisomy 21 is typically not inherited.

A small percentage of people with Down syndrome may have an extra chromosome 21 in some of their cells, but not all of them (this is called mosaicism), or they may have an extra chromosome 21 attached to another chromosome (this is called translocation).¹ Down syndrome caused by a translocation is rare, but could be inherited. A chromosome analysis (a blood test) can identify if a person with a family history of Down syndrome has a chromosome translocation that increases the risk of having a baby with Down syndrome.

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Is it possible to test for Down syndrome during pregnancy?
It is possible to diagnose Down syndrome during pregnancy through prenatal diagnostic procedures such as amniocentesis or CVS. Because amniocentesis and CVS pose some degree of risk to the pregnancy, they are generally reserved for those situations when the chance for diagnosing a chromosome abnormality is at least as high as the complication rate.

Parents might consider amniocentesis or CVS when a mother's age is 35 years or older at delivery, or a screening test done during pregnancy identifies an increased chance for Down syndrome in the fetus. Two of the most common screening tests are maternal serum screening and ultrasound. A screening test identifies those women who have an increased likelihood of having a baby with Down syndrome by identifying certain risk factors associated with the disorder. It is important to remember that having a risk factor does not mean that a baby has Down syndrome. Screening tests do not diagnose Down syndrome, they only identify those women who may benefit from additional testing.

Maternal serum screening has many other names, such as AFP and the triple screen. Ultrasound may also be useful in screening for Down syndrome. There are some signs that may be seen on a thorough ultrasound (sometimes called level II or targeted ultrasound) that may indicate there is an increased chance for Down syndrome. Many of these are minor differences in the development of an otherwise normal baby.⁴ Each ultrasound sign shows up at a different stage in pregnancy and causes a different level of concern. If any of these signs are present, they should be discussed in detail with a genetic counselor, doctor or qualified health care provider.

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The choice to have any type of screening or prenatal diagnosis procedure is very personal. It is important to fully understand the benefits and limitations of each of the options before making a decision. Genetic counselors are specially trained to guide couples through these options to a decision that seems most appropriate for them. If you would like more information about the availability of genetic counselors in your area, visit the National Society of Genetic Counselors Web Site.

Where can I get more information about Down syndrome?

National Down Syndrome Congress

National Down Syndrome Society

National Association for Down Syndrome

References:

1. Thompson MW, McInnes RR, Willard HF. Thompson & Thompson Genetics in Medicine. 5th ed. Philadelphia, Pa: W.B. Sounders Company, 1991.
2. Jones, KL. Smith's Recognizable Patterns of Human Malformation. 4th ed. Philadelphia, Pa; W.B. Saunders Company; 1988.
3. March of Dimes Birth Defects Foundation. Public Health Education Information Sheet: Down Syndrome. White Plains, NY; MOD; 1993.
4. Sanders RC, ed. Structural Fetal Abnormalities: The Total Picture. St. Louis, Mo: Mosby; 1996.
   
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