Tufts OpenCourseware
Authors: John Morgan, Bonnie F. Zimble
Down Syndrome:

Definition, Incidence and Other Facts

Special Care in Dentistry
John Morgan, DDS
Tufts University School of Dental Medicine, 2008

Study Questions:

  1. Define Down syndrome.

  2. Describe the three types of Down syndrome. Which is the most common?

  3. Explain how maternal age affects the incidence of Down syndrome.

  4. What is the current age expectancy of persons with Down syndrome?

What is Down syndrome?

According to the National Association for Down Syndrome: 

“Down syndrome is a genetic condition that causes delays in physical and intellectual development. It occurs in approximately one in every 800 live births. Individuals with Down syndrome have 47 chromosomes instead of the usual 46. It is the most frequently occurring chromosomal disorder. Down syndrome is not related to race, nationality, religion or socioeconomic status. The most important fact to know about individuals
with Down syndrome is that they are more like others than they are different.”

Click here to visit the National Association for Down Syndrome web site and read more about Down syndrome. You can print PDF or Power Point copies of any document of interest! 

There are three types of Down syndrome: trisomy 21, translocation and mosaicism. Again, refer to http://www.nads.org/pages/facts.htm for a description of each type. Approximately 95% of persons with Down syndrome are of the trisomy 21 type, due to the faulty cell division that results in the baby having three #21 chromosomes, not the usual two. The remaining 5% of cases are from translocation (3%-4%) and mosaicism.

How does maternal age affect the chance of having a child with Down syndrome?  

The incidence of Down syndrome increases with the age of the mother, as you can see in the table just below. Still, 80% of children with Down syndrome are born to women who are under age 35.  

Incidence of Down Syndrome by Maternal Age

 Age of Mother

Incidence Rate

Under 25 years


30 years


35 years


40 years


Over 45 years


Other interesting statistics on Down syndrome: 

  • The father’s age is much less important than maternal age, although there appears to be an increased risk when the father is over 50.

  • There are approximately 4000 new cases of Down syndrome per year in the U.S.

  • Approximately 80% of Down syndrome embryos are miscarried during first trimester of pregnancy.

  • Down syndrome is the most common genetic cause of intellectual disability in the U.S.

  • The current average life expectancy of persons with Down syndrome is 55 years…

  • But many individuals live into their 60’s and 70’s.