Screening for Chromosomal Defects by Fetal Nuchal Translucency at 11 to 14 Weeks RENU BINDRA, VICTORIA HEATH, and KYPROS H. NICOLAIDES Harris Birthright Research Centre for Fetal Medicine, King’s College Hospital, King’s College, London, United Kingdom The first method of screening for trisomy 21, introduced in the early 1970s, was based on maternal age. Amniocentesis was offered to women aged 35 years or more; this “high- risk” group constituted 5% of the pregnant population and contained 30% of trisomic pregnancies. In the late 1980s, a new method of screening was introduced that takes into account not only maternal age but also the concentration of various fetoplacental prod- ucts (alpha-fetoprotein, estriol, and human chorionic gonadotropin [hCG]) in the mater- nal circulation at 16 weeks of gestation. This method of screening is more effective than maternal age alone and, for the same rate of invasive testing (5%), it can identify about 65% of the fetuses with trisomy 21. In the 1990s, screening by a combination of mater- nal age and fetal nuchal translucency thick- ness (NT) at 11 to 14 weeks of gestation was introduced. This method has now been shown to identify about 75% of affected fe- tuses, for a screen-positive rate of 5%. When maternal serum free beta-hCG and preg- nancy-associated plasma protein-A (PAPP- A) at 11 to 14 weeks are also taken into ac- count, the detection rate of trisomy 21 and all major chromosomal defects is about 90%. Furthermore, the development of new methods of biochemical testing, within 30 minutes of taking a blood sample, has now made it possible to combine ultrasound and biochemistry in “One-Stop Clinics for As- sessment of Risk” (OSCAR). Recent evi- dence suggests that examination of the fetal profile for the presence or absence of the na- sal bone at the 11-to-14-week scan can po- tentially improve the sensitivity of screening to >95%. 1 In addition to its value in screening for Correspondence: K. H. Nicolaides, Harris Birthright Re- search Centre for Fetal Medicine, King’s College Hospi- tal, King’s College, Denmark Hill, London SE5 8RX. E-mail: kypros@fetalmedicine.com. CLINICAL OBSTETRICS AND GYNECOLOGY Volume 45, Number 3, 661–670 © 2002, Lippincott Williams & Wilkins, Inc. CLINICAL OBSTETRICS AND GYNECOLOGY / VOLUME 45 / NUMBER 3 / SEPTEMBER 2002 661