From the American College of OB/GYN
The current method of prenatal screening for fetal aneuploidy utilizes serum biochemical markers in combination with ultrasound measurement of the nuchal translucency based on maternal and gestational age related background risks. This screening results in a detection rate of 90-93% with a false positive rate of 2-3%.
It has been found that fetal and maternal cell-free DNA (cfDNA) circulate in maternal plasma. Fetal cfDNA is primarily derived from the placenta and is cleared from the maternal blood shortly after delivery. A higher than expected percentage of a chromosome, such as chromosome 21, may indicate a trisomy of that chromosome. This technique has been shown to detect trisomy 13, trisomy 18, and trisomy 21 as early as 10 weeks gestation. No prospective studies are available, but the use of cfDNA has demonstrated detection rates for trisomy 13, trisomy 18, and trisomy 21 of approximately 98-99% with false positive rates of less than 0.5%.
Noninvasive prenatal testing using cfDNA became commercially available in late 2011. Recent ACOG Committee Opinion from December 2012 suggested that cfDNA could be offered as a primary aneuploidy screening option for women at an increased risk of fetal aneuploidy. This includes women 35 years of age or older, and increased risk of aneuploidy by first trimester of second trimester screening test, fetus with ultrasound markers of aneuploidy, a previous child with a trisomy, or a parent who is a carrier of a balanced translocation with increased risk of a trisomy.
It is important to remember that cfDNA testing detects only trisomy 13, 18, and 21 and at present does not offer other genetic information as diagnostic tests such as amniocentesis and chorionic villus sampling do. The accuracy of cfDNA testing in twins and in the low risk population is not well established. Women with positive results on cfDNA testing should be offered confirmation with diagnostic testing. Prior to offering cfDNA testing, women should be counseled regarding limitations of the test and family history should be reviewed to determine if other screening or diagnostic options are more appropriate. Maternal serum alpha-fetoprotein screening should still be offered. Second trimester ultrasound should also be performed in high-risk pregnancies to evaluate closely for structural anomalies and to look for ultrasound markers of aneuploidy as a negative cfDNA test does not ensure an unaffected pregnancy.