Down Syndrome

What is Down Syndrome?

Down syndrome (DS), also called Trisomy 21, is a condition in which extra genetic material causes delays in the way a child develops, both mentally and physically. It affects about 1 in every 800 babies.

The physical features and medical problems associated with Down syndrome can vary widely from child to child. While some kids with DS need a lot of medical attention, others lead healthy lives.

Though Down syndrome can't be prevented, it can be detected before a child is born. The health problems that can go along with DS can be treated, and there are many resources within communities to help kids and their families who are living with the condition.

What Causes It?

Normally, at the time of conception a baby inherits genetic information from its parents in the form of 46 chromosomes: 23 from the mother and 23 from the father. In most cases of Down syndrome, a child gets an extra chromosome 21 — for a total of 47 chromosomes instead of 46. It's this extra genetic material that causes the physical features and developmental delays associated with DS.

Although no one knows for sure why DS occurs and there's no way to prevent the chromosomal error that causes it, scientists do know that women age 35 and older have a significantly higher risk of having a child with the condition. At age 30, for example, a woman has about a 1 in 900 chance of conceiving a child with DS. Those odds increase to about 1 in 350 by age 35. By 40 the risk rises to about 1 in 100.

Types of Down Syndrome

There are three forms of DS - trisomy 21, translocation and mosaicism.

Trisomy 21

This is the most common form of DS. 95% of children with DS have an extra whole chromosome in every cell of their body. It can occur as a result of faulty distribution of chromosomes in either the mother's egg or the father's sperm or as a result of faulty distribution of chromosomes in the first division of the fertilised egg.

Translocation

These comprise of about 4% of cases. DS is due to the presence of an extra part, rather than the whole, of a chromosome . This occurs when the small top portions of chromosome 21 and another chromosome break off and the two remaining portions stick to one another at their exposed ends. Only chromosomes 13, 14, 15 or 22, or another chromosome 21 (14 is the most common) are involved.

Mosaicism

Occurs in about 1% of children with DS. There is an extra whole chromosome 21 in only a proportion of their body cells. The rest of their cells are normal.