Down syndrome is usually diagnosed at birth on the basis of the typical facial features, hypotonia, and single palmar crease. Down syndrome is the most common chromosomal disorder. An estimated 1 in 800 infants are born with Down syndrome. Down syndrome is also a common cause of miscarriage. Down syndrome is a genetic disorder that results in varying degrees of physical and mental retardation. The condition varies in severity, causing developmental problems that range from mild to severe. The disorder occurs as a result of extra genetic material.

In most people, the genes are contained on 23 pairs of chromosomes, for a total of 46 chromosomes. Most people with Down syndrome have an extra copy of the No. 21 chromosome, for a total of 47 chromosomes.

Signs and symptoms 

Children with Down syndrome have a distinct facial appearance. Though not all children with Down syndrome have the same features, some of the more common features are:

• Flattened facial features

• Protruding tongue

• Small nose

• Upward slanting eyes

• Unusually shaped ears

Children with Down syndrome may also have:

• Poor muscle tone

• A single crease in the palm

• Relatively short fingers

• Excessive flexibility

Infants born with Down syndrome may be of average size, but typically they grow slowly and remain smaller than other children of similar age.

Causes 

Human cells normally contain 23 pairs of chromosomes. One chromosome from each pair comes from the father, the other from the mother. Down syndrome is caused by one of three types of abnormal cell division that involves the 21st chromosome. These abnormalities include:

• Trisomy 21. Faulty chromosome division during formation of the egg or sperm results in an embryo with three rather than two chromosome 21s. This accounts for 92 percent of all cases of Down syndrome.

• Translocation. During chromosome division a part of chromosome 21 separates and attaches to another chromosome. About 3 percent to 4 percent of all cases are due to translocation.

• Mosaicism. Abnormal cell division in the early life of an embryo results in some cells with the normal 46 chromosomes, while others have 47, creating a mosaic of normal and abnormal cells. This accounts for 2 percent to 4 percent of all cases.

There are no known behavioral or environmental factors that cause Down syndrome.

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