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Down Syndrome - FAQ

These frequently asked questions are adapted from Down Syndrome Frequently Asked Questions

From Parents/Families

Is Down syndrome inherited?

Only 3 to 5% of cases are inherited; the rest arise as an accident of chromosome arrangement after conception and during cell division (meiosis). In 90% of cases, trisomy 21 is due to nondisjunction of the maternal chromosome; in 10% of cases it is due to nondisjunction of the paternal chromosome.

If maternal age over 35 years is a risk factor for having babies with DS, why are more than half of all babies with DS born to women under 35 years?

While it is much more common for babies with DS to be born to women over 35 years of age, women under 35 have a higher overall birth rate. No risk factors have been found yet for women under 35 years of age, but several research groups are looking at this question.

How likely is a person to have a child with Down syndrome if he/she has a sibling with DS?

For the vast majority of people, having a sibling with DS does not increase one's risk of having a child with DS. That's because 95% of all cases of DS are not inherited. The chromosomal test on the person with DS will show how likely it is to be an inherited case.

Authors

Compiled and edited by: Catherine Jolma MD, 10/2009
Content Last Updated: 1/2010