Cause

Down syndrome is caused by abnormal cell division in early embryo development. Normally, a child inherits 46 chromosomes, 23 from each parent. Each chromosome carries DNA, called genes, which tell how the brain and body should develop.

But a fetus with Down syndrome has extra or abnormal chromosomes. Having extra genetic material changes the way the brain and body develop. The type of Down syndrome depends on how many cells have the extra or abnormal chromosomes.

Medical experts believe the cell changes most often start in a woman's egg before or at conception. Less often, the error occurs in sperm at conception. It is not known what causes the cells to divide abnormally. One type of Down syndrome, called translocation-type, may be passed down through families (inherited).

Symptoms

There are more than 50 features of Down syndrome. But not every person with Down syndrome has all the same features or health problems. Some features and problems are common.

Body shape and size

• Short stature (height). A child often grows slowly and is shorter than average as an adult.
• Weak muscles (hypotonia) throughout the body. Weak belly muscles also make the stomach stick out.
• A short, wide neck. The neck may have excess fat and skin.
• Short, stocky arms and legs. Some children also have a wide space between the big toe and second toe.

Face shape and features

• Slanted eyes. Tissue may also build up on the colored part of the eye (iris). But the child's vision is not affected by this buildup.
• A nasal bridge that looks pushed in. The nasal bridge is the flat area between the nose and eyes.
• Small ears. And they may be set low on the head.
• Irregularly shaped mouth and tongue. The child's tongue may partly stick out. The roof of the mouth (palate) may be narrow and high with a downward curve.
• Irregular and crooked teeth. Teeth often come in late and not in the same order that other children's teeth come in.

Health problems

Health problems related to Down syndrome, such as:

• Intellectual disability. Most children with Down syndrome have mild to moderate cognitive disability.¹
• Heart defects. About half of the children who have Down syndrome are born with a heart defect.
• Hypothyroidism, celiac disease, and eye conditions.
• Respiratory infections, hearing problems, or dental problems.
• Depression or behavior problems associated with ADHD or autism.