Kids With Down Syndrome: What Causes It?

Down syndrome, which occurs in one of every 700 to 800 births worldwide, is caused by the presence of an extra 21st chromosome in the cells of the body. It starts at conception, when either the sperm or the egg, through an error of cell division, has the extra chromosome at the time of fertilization. We're not sure why the initial error occurs, though we know that the risk increases with the age of the mother.

In Down syndrome, when the fertilized egg divides as the embryo forms, all of the cells have the extra copy of the 21st chromosome. This is why the condition is also called "trisomy 21," meaning three of the 21st chromosome. This produces typical or "simple" Down syndrome, which makes up 95 percent of all cases of DS.

The other five percent of cases are made up of two other types of Down syndrome. One type, translocation Down syndrome, occurs when there are two breaks in the chromosome material and the extra chromosome material becomes stuck onto one of the 21st chromosomes. This type of Down syndrome can sometimes be an inherited condition.

The third type occurs when only part of the cells of the body have the extra 21st chromosome, and is called mosaic Down syndrome, like the mosaic style of art in which a picture is made up of different colors of tiles. In Down syndrome, mosaicism means that some cells of the body have trisomy 21, and some have the typical number of chromosomes.

Doctors diagnose which type of Down syndrome through blood testing of the child.