Screening for Fetal Aneuploidy

Aneuploidy is defined as having one or more extra or missing chromosomes, leading to an unbalanced chromosome number in a cell. Because each chromosome consists of hundreds of genes, the loss or gain of large chromosomal segments disrupts significant amounts of genetic material and often results in a nonviable pregnancy or offspring that may not survive after birth. In the case of a surviving newborn, congenital birth defects; failure to thrive; and functional abnormalities, including mild-to-severe intellectual disability, infertility, and shortened lifespan, may occur. Although chromosomal abnormalities occur in approximately 1 in 150 live births (1), the prevalence is greater earlier in gestation because aneuploidy accounts for a large proportion of early pregnancy loss. The incidence of fetal aneuploidy increases as a woman ages (Table 1) but can affect any woman regardless of age and is not related to race or ethnicity. Other factors that increase the risk of fetal aneuploidy include a history of a prior aneuploid fetus and the presence of fetal anomalies. Autosomal trisomies are the most common aneuploidies that are not related to sex chromosome disorders. Down syndrome (trisomy 21) is the most common of these, with a prevalence of approximately 1 in 800 live births Screening for Fetal Aneuploidy