A New Era in Prenatal Diagnosis: The Use of Cell-Free Fetal DNA in Maternal Circulation for Detection of Chromosomal Aneuploidies

Abstract

Prenatal screening for chromosomal aneuploidies is a fundamental part of routine obstetric care in most countries. Typically, maternal age, weight, ethnicity, serum biomarkers (including pregnancy-associated plasma protein A, human chorionic gonadotropin, α-fetoprotein, inhibin A, and estriol), and sonographic features (i.e., nuchal translucency) are included in a risk algorithm to determine the probability of the fetus being affected. Pregnant women identified as at high risk according to the prenatal screen can then undergo invasive procedures, such as amniocentesis and chorionic villus sampling, to confirm the diagnosis. Current prenatal-screening methods are able to identify approximately 90% of pregnancies affected by trisomy 21 (Down syndrome) at a false-positive rate of approximately 5%. Given that the prevalence of chromosomal aneuploidies is generally quite low, a false-positive rate of 5% means that a large number of women with unaffected pregnancies undergo invasive procedures, putting the fetus at an unnecessary risk for miscarriage. The discovery of fetal cell-free DNA in the plasma of pregnant women 14 years ago opened up the possibility of identifying chromosomal abnormalities noninvasively, through a single blood sample. Approximately 10% of cell-free DNA in the maternal circulation is of fetal origin, and this property was initially exploited to determine rhesus D status and the sex of the unborn fetus. The advent of next-generation DNA sequencing, however, has allowed prenatal detection of chromosomal aneuploidies, including trisomy 21, from maternal blood. In brief, the proportion of chromosome 21 DNA molecules in maternal plasma is measured directly; an increase above a predetermined threshold is indicative of trisomy 21. The clinical performance of this noninvasive prenatal test has been promising, with recent clinical studies having shown a diagnostic sensitivity of 100% and a diagnostic specificity of 98%–99%, compared with full karyotyping by invasive means. When used as a second-tier screening procedure, this technology also has the …