Down syndrome is a genetic condition that causes mental retardation and some specific physical characteristics. It was named after John Langdon Down, the 19^th century physician who first described it. The condition occurs in about one in every 800 newborns, according to the U.S. Centers for Disease Control and Prevention (CDC), and occurs equally among all races and ethnic groups.

Down syndrome is caused by the presence of an extra chromosome. Most cells in the human body have 46 chromosomes in 23 pairs. The mother and father each contribute one-half of each pair. However, people with Down syndrome have triplication of chromosome 21 (an extra copy or extra genetic material of the smallest of these chromosomes, chromosome 21) The cause of this chromosomal abnormality is not known. However, the risk of having a child with Down syndrome increases as a parent ages.

The characteristics, features and potential complications of Down syndrome vary greatly among patients. Down syndrome is commonly associated with a number of physical features, including a flat facial profile, upward slanting eyes and a single crease along the center of the palms. Various medical issues, including an increased susceptibility to infection, poor muscle tone, congenital heart defects and eye, ear, breathing and digestive problems, are also generally associated with Down syndrome.

The extra genetic material present in Down syndrome also results in developmental delays, including mild to moderate mental retardation. Infants born with Down syndrome are usually born an average size, but tend to grow more slowly than others. Because of this, most people with Down syndrome will be smaller than their age peers throughout life. Other developmental delays include delays in sitting up, walking, talking and toilet training. Early intervention with speech, physical and occupational therapy can help overcome these delays. Children and adolescents with Down syndrome may attend regular schools, although some may still need special classes to help with learning disabilities.

Down syndrome is often detected before birth. Prenatal screening tests may be used to determine the chance that a particular fetus has Down syndrome. A positive result on a screening test may lead to the use of a diagnostic test such as amniocentesis or chorionic villus sampling. Prenatal diagnosis of Down syndrome may give parents the time to decide to terminate the pregnancy or to adjust to the upcoming challenges of raising a child with Down syndrome. After an infant is born, Down syndrome can usually be diagnosed by a physical examination.

There is no cure for Down syndrome and no known way to prevent it. Though the condition itself cannot be treated, the health problems that are often associated with it can be.