Trisomy is a genetic condition characterized by an extra copy of a chromosome, resulting in a total of 47 chromosomes instead of the usual 46. The term "trisomy" means that the affected person has three copies of one of the chromosomes instead of two. The most common types of trisomy are:

• Trisomy 21: Also known as Down syndrome, this occurs when there is an extra copy of chromosome 21.
• Trisomy 18: Also known as Edwards syndrome, this occurs when there is an extra copy of chromosome 18.
• Trisomy 13: Also known as Patau syndrome, this occurs when there is an extra copy of chromosome 13.

Trisomy disorders are caused by an alteration in the number or genetic structure of chromosomes, and the addition of an extra chromosome usually occurs spontaneously during conception. The cause of this is unknown, and prevention is not possible. The most important risk factor for trisomy conditions is maternal age, with women in their late 30s and 40s having a higher chance of trisomy conditions occurring. Children affected by trisomy usually have a range of birth anomalies, including delayed development and intellectual disabilities. Treatment for children born with trisomy includes surgery to repair any physical abnormalities, educational support, speech, behavioral and physical therapy, and medicines to help symptoms of related medical conditions that develop over time.