Down's syndrome is a common cause of birth defects and mental retardation. Prenatal screening and diagnosis of Down's syndrome is important to any pregnant woman interested in the health of her fetus and is of particular concern to the growing number of advanced maternal age women who are at increased risk of an affected pregnancy. Prenatal screening tests, performed in the first and second trimester, are based on maternal age, serum tests and fetal ultrasound. Definitive diagnosis requires amniocentesis or chorionic villus sampling. Hopefully, these invasive tests can, one day, be replaced with safer methods of obtaining fetal cells. Molecular genetic techniques are augmenting traditional chromosome analysis, broadening the range of identifiable genetic disorders and allowing earlier results.