Down syndrome is a variable combination of congenital malformations caused by trisomy 21. It is the most commonly recognized genetic cause of mental retardation, with an estimated prevalence of 9.2 cases per 10,000 live births in the United States. Because of the morbidity associated with Down syndrome, prenatal screening and diagnostic testing for this condition are offered as optional components of prenatal care. The purpose of prenatal screening and diagnosis is to identify those women at increased risk for an affected pregnancy and to maximize the options available to their families. Screening for aneuploidy identifies women whose fetuses are at increased risk for Down syndrome, trisomy 18, or trisomy 13. Screening and invasive diagnostic testing for aneuploidy should be offered to all women who present for prenatal care before 20 weeks gestation, regardless of maternal age. Screening for aneuploidy identifies women whose fetuses are at increased risk for Down syndrome, trisomy 18, or trisomy 13. It is not recommended that all pregnant women be screened, but it is recommended that all pregnant women be offered screening. Physicians are ethically obligated to fully inform our patients of their health care options, including prenatal testing. It is entirely up to the patient to decide whether or not she wishes to be screened for fetal chromosomal abnormalities without judgment from the physician. The goal is to offer screening tests with high detection rates and low false-positive rates and also to offer patients diagnostic testing options if the screening test indicates an increased risk for having a child with a chromosomal abnormality.