The term trisomy is used to describe the presence of three chromosomes, rather than the usual matched pair of chromosomes. For example, if a baby is born with three #21 chromosomes, rather than the usual pair, then the baby would be said to have "trisomy 21." Trisomy 21 is also known as Down syndrome. Other examples of trisomy include syndromes like trisomy 18 and trisomy 13. Again, trisomy 18 or trisomy 13 simply means the child has three copies of the #18 chromosome (or of the #13 chromosome) present in each cell of the body, rather than the usual pair.