Down Syndrome

Down syndrome, named after physician John Langdon Down, is the chief genetic cause of mild to moderate mental retardation. It occurs in 1 in 660 births, affecting all ethnic and socioeconomic groups.

Down syndrome has several forms. In the most common form, called trisomy 21, an extra chromosome 21 is found in every cell of a person with the condition. Another form of Down syndrome is called mosaic trisomy 21. This occurs in 2 percent to 4 percent of Down cases. The extra chromosome is present in some of the cells, but not all of them. A third form of Down is called translocation trisomy 21. It occurs in about 4 percent of Down cases, and involves a bit of chromosome 21 attached to another chromosome (most frequently chromosome 14).

The cause of the extra chromosome appears to be a random event. It is not caused by something the parents have done or by an environmental factor. Still, the incidence of Down syndrome increases with the mother's age. In the majority of Down cases, the extra chromosome 21 comes from the mother. The likelihood that a woman age 35 will have a baby with Down syndrome is 1 in 400; at age 40, it's 1 in 105; at 44, it's 1 in 35. These figures may vary, depending on the source.

Prenatal testing is available for Down syndrome. An initial screening blood test (“triple screen”) can measure three markers that raise the suspicion of Down syndrome: serum alpha feto-protein, chorionic gonadotropin and unconjugated estriol.

If this screening suggests that Down syndrome is possible, chromosomal studies can then be done, which can absolutely make the diagnosis.  Amniocentesis, in which a sample of the mother's amniotic fluid is taken, can be done in the 14th to 18th week of pregnancy. Chorionic villus sampling (CVS) can be done earlier, in the ninth to 11th week of pregnancy, but carries a greater risk of miscarriage. In CVS, a tiny amount of the tissue that will eventually become the placenta is extracted so that it can be analyzed.

Percutaneous umbilical blood sampling (PUBS) is the most accurate test, and is usually done to confirm the results of either amniocentesis or CVS. It cannot be done until the 18th to 22nd week of pregnancy.

What to Do

Talk to your doctor if you are pregnant. Your doctor can discuss possible prenatal testing, particularly if you are age 35 or older, or have a previous child with Down syndrome.

Special Concerns for Children With Down Syndrome

A child born with Down syndrome may have some or all of the characteristics listed under Signs/Symptoms. Children with Down syndrome can have a wide range of abilities. Some children and adults with Down syndrome are basically healthy; others may have medical problems, including hypothyroidism, hearing loss, congenital heart disease and vision disorders. Children with Down syndrome are at higher risk for developing leukemia and defective development of the spinal cord. Youngsters with Down syndrome also are more likely to die from heart problems and from infectious diseases because of abnormalities in their immune systems. A child with Down syndrome will be developmentally delayed, taking longer to reach developmental milestones.

Self-care Steps for Down Syndrome

• Find a support group for parents of Down syndrome children.