Advances in Prenatal Screening: Nuchal Translucency Ultrasonography in the First Trimester

After completing this article, readers should be able to: 1. Define the association between nuchal translucency and Down syndrome. 2. Describe the role of nuchal translucency-based ultrasonography in prenatal screening for Down syndrome in singleton and multiple gestations. 3. Characterize the criteria for good nuchal translucency ultrasonographic technique. 4. Identify the limitations of implementing first trimester screening in the general obstetric population. Prenatal screening for Down syndrome and other aneuploidies has advanced significantly since its advent in the 1980s. Antenatal screening for Down syndrome began by selecting women older than age 35 years as candidates for amniocentesis. Maternal serum screening for Down syndrome in the second trimester started in the mid-1980s, with low levels of the analyte alpha-fetoprotein (AFP) associated with an increased risk of fetal Down syndrome. The panel of serum analytes has expanded considerably and generally includes four markers, known as the “quad test,” comprised of AFP, human chorionic gonadotropin (hCG), unconjugated estriol, and most recently, inhibin-A. Second-trimester multiple-marker screening and maternal age remain the standard-of-care in prenatal screening for Down syndrome throughout the United States today, although they can detect only 55% (the double screen) to 76% (the quad test) of Down syndrome cases, with a 5% false-positive rate (FPR). Much emphasis in Down syndrome screening research has been placed on finding protocols that increase the detection rate and make screening available at an earlier time in pregnancy. The most significant advance in this front came in the early 1990s, with the ultrasonography marker nuchal translucency (NT) at 10 to 14 weeks’ gestation, in combination with maternal serum markers of free beta-hCG and pregnancy-associated plasma protein A (PAPP-A). This review focuses on ultrasonographic screening for Down syndrome in the first trimester. Fetal NT refers to the normal subcutaneous fluid-filled space between the back of the neck and the overlying skin (Fig. …