In the past 2 decades, the second trimester of pregnancy has been the most common time for prenatal diagnosis of fetal anomalies and chromosomal aneuploidies. More recently, screening for and diagnosis of chromosomal abnormalities are increasingly being performed in the first trimester. With improvements and technological advances in ultrasound, it is now possible to identify many fetal structural anomalies at 11 to 13 6/7 weeks' gestation. At 10 to 11 weeks' gestation, biochemical markers in serum-PAPP-A, free beta-hCG, AFP, and uE3-combined with sonographic measurement of nuchal translucency and the presence/absence of the nasal bone can achieve a Down syndrome detection rate of 97.5% at a false-positive rate of 5%. Structural anomalies of the central nervous system, and the cardiac, renal, and gastrointestinal tracts can now be diagnosed by either transabdominal or transvaginal scanning, achieving detection of up to 80% of CNS anomalies by 13 weeks' gestation. In future, the emphasis in prenatal diagnosis will likely be in the first trimester.
Target audience: Obstetricians & Gynecologists, Family Physicians.
Learning objectives: After completion of this article, the reader should be able to explain the rationale for first trimester combined ultrasound and serum analyte screening for fetal Down syndrome, describe the fetal anatomic structures that can be seen and evaluated in the first trimester, provide patient counseling about the relative benefits of genetic amniocentesis versus chorionic villous sampling, and discuss the application of Doppler technology to the evaluation of a first trimester fetus.