Normally, a baby inherits 23 chromosomes from each parent, for a total of 46. Each chromosome matches the corresponding chromosome from the other parent, forming 23 numbered pairs.

Biological errors can happen during the early stages of cell division, causing some fetuses to develop with 47 chromosomes: Instead of 23 matched pairs, they have 22 pairs plus one set of three, a chromosomal abnormality called trisomy.

Most fetuses with trisomy are miscarried during the first trimester. Most of those who survive have Down syndrome, also known as trisomy 21 because it's caused by an extra chromosome 21.

While all babies with Down syndrome have some degree of mental retardation, it's severe in only about 10 percent. Many people with Down syndrome have rich and fulfilling lives into adulthood.

Other babies with chromosomal abnormalities have trisomy 13 or 18, which are almost always associated with profound mental retardation and other congenital malformations. These babies rarely live more than a few days or months.

The risk of having a baby with trisomy increases with the mother's age, which is why healthcare providers suggest genetic testing for women who will be 35 or older on their due date.