irst-trimester screening for Down syndrome has become a commonly used approach in prenatal genetic diagnosis. The drive behind the development of earlier, reliable methods of genetic screening and risk assessment is the ability to provide the parents with more options at an earlier gestational age. Currently, this involves the sonographic evaluation of fetal nuchal translucency (NT) in combination with maternal serum levels of free beta-hCG and PAPP-A to provide a risk assessment for Down syndrome.1 This is referred to as combined first-trimester screening. Based on a survey of US maternal-fetal medicine specialists in 2001, of the 543 respondents, 46% used NT sonography and 27% used first-trimester maternal serum screening for Down syndrome.3 The term "nuchal translucency" refers to the fluid-filled space between the back of the fetal neck and the overlying skin.4 It has been noted that fetuses with Down syndrome, as well as other forms of aneuploidy, have increased edema in this area, resulting in an increased NT measurement.4 Based on seminal work by Nicolaides and others, measurement ofNT in fetuses between 11 and 14 weeks has been used since the early 1990's to provide earlier risk assessment of fetuses with aneuploidy.5 Initial results indicated detection rates comparable to second-trimester maternal serum screening.' However, three recent large trials, including the US-based Firstand SecondTrimester Evaluation of Risk (FASTER) trial, the North American-based First Trimester Maternal Serum Biochemistry and Ultrasound Fetal Nuchal Translucency Screening (BUN) Study, and the European-based Serum, Urine, and Ultrasound Screening Study (SURUSS) Trial have demonstrated that combined screening in the first trimester outperforms both NT and first-trimester serum screening when performed separately, with detection rates for Down syndrome ranging from 79%87% at a false positive rate of 5% 6 This compares favorably with performance of second-trimester serum screening, also referred to as the "quad" screen, which utilizes maternal serum levels of unconjugated estriol, free human chorionic gonadotropin, alpha-fetoprotein, and inhibin A, with a detection rate of 81%.7 Furthermore, the detection rate for trisomy 18 with the combined screen was comparable to the quad screen.8 There are still a number of issues which limit the use ofNT. The technical expertise required to obtain reliable and reproducible images has proven challenging. Furthermore, patients must have access to providers who are capable of performing first-trimester chorionic villus sampling when a patient screens positive. There is still no consensus as to the most efficient or