Down syndrome is a genetic condition in which a person has 47 chromosomes instead of the usual 46.

In most cases, Down syndrome occurs when there is an extra copy of chromosome 21. This form of Down syndrome is called Trisomy 21. The extra chromosome causes problems with the way the body and brain develop.

Down syndrome is the most common single cause of human birth defects.

Down syndrome symptoms vary from person to person and can range from mild to severe. However, children with Down syndrome have a widely recognized characteristic appearance.

The head may be smaller than normal and abnormally shaped. For example, the head may be round with a flat area on the back. The inner corner of the eyes may be rounded instead of pointed.

Common physical signs include:

• Decreased muscle tone at birth
• Excessive skin at the nape of the neck
• Flattened nose
• Separated sutures (joints between the bones of the skull)
• Single crease in the palm of the hand
• Small ears
• Small mouth
• Upward slanting eyes
• Wide, short hands with short fingers
• White spots on the colored part of the eye (Brushfield spots)

Physical development is often slower than normal. Most children with Down syndrome never reach their average adult height. Children may also have delayed mental and social development. According to the National Institute of Child Health and Human Development, most persons with Down syndrome have mild to moderate mental retardation.

Many different medical conditions are seen in babies born with Down syndrome, including

• Birth defects involving the heart such as an atrial septal defect or ventricular septal defect
• Eye problems such as cataracts
• Gastrointestinal blockages such as esophageal atresia and duodenal atresia
• Hearing problems
• Hip dislocation
• Sleep apnea
• Underactive thyroid (hypothyroidism)

Children with Down syndrome also have a higher risk for acute lymphocytic leukemia.

A doctor can often make an initial diagnosis of Down syndrome at birth based on how the baby looks. The doctor may hear a heart murmur when listening to the chest with a stethoscope.

A blood test can be done to check for the extra chromosome and confirm the diagnosis. See: Chromosome studies

Other tests that may be done include:

• Echocardiogram
• ECG
• X-rays of the chest and gastrointestinal tract

Early and massive vomiting may be a sign of a gastrointestinal blockage. The blockage may also be discovered when a health care provider is unable to pass a tube from the nose into the stomach. See: Nasogastric or endotracheal intubation