Approximately 1.5% of liveborn infants will have a major congenital anomaly that is genetic in origin and genetic abnormalities are the most common cause of spontaneous abortion. Screening and diagnosis of genetic abnormalities is an important part of routine prenatal care. Prenatal screening and diagnosis can detect numerical chromosomal abnormalities called aneuploidy, structural chromosomal abnormalities like deletions or duplications, and single gene mutations. These genetic abnormalities may be spontaneous, arising in the fetus de novo, or may be inherited from parents who are carriers or who are affected themselves. Patients may also opt to undergo testing to determine their carrier status for heritable genetic conditions as part of preconception planning. This chapter will review carrier screening, preimplantation genetic testing, and screening for and diagnosis of fetal genetic conditions with emphasis on aneuploidy testing options, patient counseling regarding test choice, and interpretation of testing results.