Down syndrome is a genetic condition caused by abnormal cell
division in the egg, sperm, or fertilized egg. This results in
an extra or irregular chromosome in some or all of the body's
cells, causing varying levels of intellectual disability and
physical problems. Down syndrome is also called trisomy 21, for
the specific chromosome that has the abnormality. A person with
Down syndrome has three copies of chromosome 21. Normally, a
person has two copies. Down syndrome usually can be detected
during pregnancy or soon after birth. Chromosome (karyotype)
tests and how a baby looks can help make a diagnosis. Babies
usually have distinctive facial characteristics, such as upward-
sloping eyes and a flattened nose. People with Down syndrome
have an increased risk of being born with or developing health
problems. For example, some babies with Down syndrome are born
with heart, intestinal, ear, or respiratory defects. These
health conditions often lead to other problems, such as
respiratory infections, sleep apnea, or