In most cases, Down syndrome is caused by an extra chromosome 21. It is the most common single cause of human birth defects, with an occurrence in 1 out of every 660 births. Children with Down syndrome have a widely recognized characteristic appearance. The head may be smaller than normal (microcephaly) and abnormally shaped. Prominent facial features include a flattened nose, protruding tongue, and upward slanting eyes. The inner corner of the eyes may have a rounded fold of skin (epicanthal fold) rather than coming to a point. The hands are short and broad with short fingers and often have a single crease in the palm (simian crease). Retardation of normal growth and development is typical and most affected children never reach average adult height. Congenital heart defects are frequently present in Down syndrome children. Early mortality is often a result of cardiac abnormalities. Gastrointestinal abnormalities such as esophageal atresia (obstruction of the esophagus) and duodenal atresia (obstruction of the duodenum) are also relatively common. Obstruction of the gastrointestinal tract may require major surgery shortly after birth. Children with Down syndrome also have a higher-than-average incidence of acute lymphocytic leukemia (ALL).
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