First-Trimester Screening David A. Nyberg, MD a, *, Jon Hyett, MD b , Jo-Ann Johnson, MD, FRCSC c , Vivienne Souter, MD d All patients have a 2% to 3% risk of birth defects, regardless of their prior history, family history, ma- ternal age, or lifestyle [1]. Chromosome abnormal- ities account for approximately 10% of birth defects, but are important because of their high mortality and morbidity. Trisomy 21 (Down syn- drome) is the most common serious chromosome abnormality at birth, occurring in approximately 1 of 500 pregnancies in the United States. The actual risk varies with maternal and gestational age and whether there is a history of previous pregnancies affected by chromosomal abnormality, although, as with other birth defects, all patients are at risk for fetal Down syndrome. A detailed fetal anatomic survey performed at 18 to 22 weeks remains the primary means for detect- ing the majority of serious ‘‘structural’’ birth defects; however, first-trimester screening at 11 to 14 weeks has developed into the initial screening test for many patients. A wealth of information can be obtained at this time, including detection of many structural defects, as well as screening for fetal aneu- ploidy, including Down syndrome. The major ad- vantage of first-trimester screening is the earlier gestational age of detection so that diagnostic test- ing (chorionic villous sampling [CVS] or genetic amniocentesis) can be made available for patients considered at highest risk for chromosome abnor- malities. First-trimester screening can also help identify patients at increased risk for a variety of other abnormalities, including cardiac defects, that may be seen later. In this way, first-trimester screening can help triage patients for subsequent testing. Older screening methods relied on clinical risk factors, particularly maternal age, to determine which patients might benefit from a diagnostic in- vasive test for fetal aneuploidy; however, maternal age alone is a poor screening method for determin- ing who is at risk for chromosome abnormalities. RADIOLOGIC CLINICS OF NORTH AMERICA Radiol Clin N Am 44 (2006) 837–861 This article was originally published in Ultrasound Clinics 1:2, April 2006. a Fetal and Women’s Center of Arizona, 9440 E. Ironwood Square Drive, Scottsdale, AZ 85258, USA b Maternity Services, Royal Brisbane Women’s Hospital, Butterfield Street, Herston GLD 4006, Australia c Department of Obstetrics and Gynecology, Calgary, AB, Canada d Good Samaritan Medical Center, 1111 East McDowell Road, Phoeniz, AZ 85006, USA * Corresponding author. E-mail address: nyberg@u.washington.edu (D.A. Nyberg). - First-trimester aneuploidy screening - Screening strategies First-trimester combined screen First-trimester combined screen plus other ultrasound markers First-trimester screening followed by second-trimester biochemistry First-trimester screening followed by second-trimester ultrasonography - Other advantages of first-trimester screening Other chromosome abnormalities Birth defects in euploid fetuses who have increased nuchal translucency Twins and multiple gestations Structural defects detected during the first trimester - Summary - References 837 0033-8389/06/$ – see front matter ª 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.rcl.2006.10.017 radiologic.theclinics.com