What Causes Down Syndrome?

A fertilized egg normally has 23 pairs of chromosomes.  However, most of the people who have Down Syndrome have an extra (or third) copy of chromosome 21. Having this extra copy of chromosome 21 changes the body’s and brain’s normal development.

In most cases Down Syndrome is caused by a random error in cell division that happens during formation of the mother’s egg or father’s sperm. Because of this error, when fertilization does take place, the embryo has an additional third chromosome 21 or “trisomy 21.”

It is not believed that Down Syndrome is a result of the parents’ behavior or environmental reasons.

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National Down Syndrome Society