Trisomy 21 is another name for Down syndrome, a birth defect affecting chromosome number 21, according to the Centers for Disease Control and Prevention. A person normally has 23 pairs of chromosomes for a total of 46. Trisomy 21 happens when a person has an extra copy of chromosome 21.
The extra chromosome causes developmental delays both intellectually and physically. The severity of those symptoms ranges from mild to moderate, notes Mayo Clinic. In some people, Down syndrome causes additional health problems such as heart defects.
Trisomy 21 typically causes distinctive facial features, says Mayo Clinic. Facial features often appear flattened with slanted eyes, a protruding tongue, small ears, a small head and a short neck. Other physical symptoms include poor muscle tone, excessive flexibility, shortness, short fingers, and small feet and hands. While typically average in size at birth, a child with trisomy 21 often grows more slowly than peers.
Health problems associated with Down syndrome often include hearing loss, ear infections, eye infections, obstructive sleep apnea and heart defects, states the CDC. Less common problems include intestinal blockage, hip dislocation, anemia and thyroid disease. Trisomy 21 never goes away, but babies diagnosed with the syndrome receive services to support development. Most states have early intervention programs with various therapy services.