The most common trisomy is called Down syndrome, a trisomy of chromosome pair 21, and it leads to mental retardation and various physical problems. Examining the chromosomes also allows the sex to be determined, which may be of particular interest to women who are carriers of sex-linked disorders, such as hemophilia or Duchenne muscular dystrophy. It is also possible to detect about 70 metabolic disorders. The tests for metabolic disorders, however, are only done if family history warrants, and will not be done for women being referred for amnio due to age or results on the AFP test or Down's screen. If there is no history of genetic disease in the family, a genetic analysis will not be performed, only a chromosomal analysis.
Chromosomal analysis is performed at most large hospitals and some private labs.
Genetic analysis is only performed in a few labs in the country and is significantly more expensive. Tay Sachs, which is common among Eastern European Ashkenazi Jews and French Canadians, can be detected by amniocentesis. We don't do it in my hospital because our ethnic mix doesn't warant the development of the expertise. Amniocentesis will not guarantee you a normal child. It cannot detect most non-chromosomal genetic defects, nor can it detect defects of body structure, such as harelip, cleft palate, congenital heart disease, hypospadias, pyloric stenosis, clubfoot or congenital hip dislocation.