The term “trisomy” is used to describe the presence of three chromosomes, rather than the usual pair of chromosomes. For example, if a baby is born with three #21 chromosomes, rather than the usual pair, the baby would be said to have “trisomy 21.”

Trisomy 21, also referred to as Down Syndrome, is a chromosome abnormality. The syndrome is caused by an extra copy of chromosome 21. Trisomy 21 is a common birth defect, appearing in about one out of every 800 newborns.

Other examples of trisomy include trisomy 18 and trisomy 13. Again, trisomy 18 or trisomy 13 simply means there are three copies of the #18 chromosome (or of the #13 chromosome) present in each cell of the body, rather than the usual pair.

Trisomy 21 is a genetic disorder and is one of the most common genetic birth defects, affecting approximately one in 800 to 1,000 babies. In the United States, there are more than 350,000 individuals with Down syndrome, according to the National Down Syndrome Society. Life expectancy among adults with Down syndrome is about 55 but through advances in science, exercise, and nutrition the those numbers are significantly rising.

The name “Down syndrome” comes from the physician, Dr. Langdon Down, who first described the collection of findings in 1866. It was not until 1959 that the cause of Down syndrome (the presence of an extra #21 chromosome) was identified.