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Down Syndrome

Diagnosis And Prevention

Down syndrome can be diagnosed at birth, when the characteristic physical signs of Down syndrome are noted and chromosome analysis can also be performed to confirm the diagnosis and determine the recurrence risks.

At-risk pregnancies are referred for genetic counseling and prenatal diagnosis. Screening tests are available during a pregnancy to determine if the fetus has Down syndrome. During 14–17 weeks of pregnancy, a substance called AFP (alpha-fetoprotein) can be measured. AFP is normally found circulating in a pregnant woman's blood, but may be unusually high or low with certain disorders. Carrying a baby with Down syndrome often causes AFP to be lower than normal. This information alone, or along with measurements of two other hormones, is considered along with the mother's age to calculate the risk of the baby being born with Down syndrome.

A common method to directly determine whether the fetus has Down syndrome, is to test tissue from the fetus. This is usually done either by amniocentesis, or chorionic villus sampling (CVS). In amniocentesis, a small amount of the fluid in which the baby is floating is withdrawn with a long, thin needle. In chorionic villus sampling, a tiny tube is inserted into the opening of the uterus to retrieve a small sample of the chorionic villus (tissue that surrounds the growing fetus). Chromosome analysis follow both amniocentesis and CVS to determine whether the fetus is affected.

Once a couple has had one baby with Down syndrome, they are often concerned about the likelihood of future offspring also being born with the disorder. In most cases, it is unlikely that the risk is greater than other woman at a similar age. However, when the baby with Down syndrome has the type that results from a translocation, it is possible that one of the two parents is a carrier of that defect. When one parent is a carrier of a particular type of translocation, the chance of future offspring having Down syndrome is increased. The specific risks can be estimated by a genetic counselor.


Resources

Books

Nussbaum, R.L., Roderick R. McInnes, and Huntington F. Willard. Genetics in Medicine. Philadelphia: Saunders, 2001.

Rimoin, David, L. Emery and Rimoin's Principles and Practice of Medical Genetics. London; New York: Churchill Livingstone, 2002.

Periodicals

"Medical and Surgical Care for Children with Down Syndrome." The Exceptional Parent 25 no. 11 (November 1995): 78+.


Rosalyn S. Carson-DeWitt

Bryan R. Cobb

KEY TERMS


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Chromosomes

—The structures that carry genetic information in the form of DNA. Chromosomes are located within every cell and are responsible for directing the development and functioning of all the cells in the body. The normal number of chromosomes in humans is 46 (23 pairs).

Developmental delay

—A condition where an individual has a lower-than-normal IQ, and thus is developmentally delayed.

Egg cell

—The female's reproductive sex cell.

Embryo

—A stage in development after fertilization.

Karyotype

—The specific chromosomal makeup of a particular cell.

Mosaic

—A term referring to a genetic situation in which an different cells do not have the exact same composition of chromosomes. In Down syndrome, this may mean that some of the individual's cells have the normal 46 number of chromosomes, while other cells have an abnormal number, or 47 chromosomes.

Nondisjunction

—A genetic term referring to an event that takes place during cell division in which one of the newly created cells has 24 chromosomes and the other cell has 22, rather than the normal 23.

Sperm

—Substance secreted by the testes during sexual intercourse. Sperm includes spermatozoon, the mature male cell which is propelled by a tail and has the ability to fertilize the female egg.

Translocation

—A genetic term referring to a situation during cell division in which a piece of one chromosome breaks off and sticks to another chromosome.

Trisomy

—The condition of having three identical chromosomes, instead of the normal two.

Zygote

—The cell resulting from the fusion of male sperm and the female egg. Normally the zygote has double the chromosome number of either gamete, and gives rise to a new embryo.

Additional topics

Science EncyclopediaScience & Philosophy: Direct Variation to DysplasiaDown Syndrome - Causes And Symptoms, Treatment, Prognosis, Diagnosis And Prevention