OBGYN -- Unit 2: Obstetrics A - Normal Obstetrics

Terms in this set (80)

C. Amniocentesis is a diagnostic test that may detect Down syndrome as well as other chromosomal abnormalities. Cell-free DNA testing detects over 99% of cases of Down syndrome. The first trimester screen, which consists of a nuchal translucency and maternal serum PAPP-A and beta-hCG, yields an 85% detection rate for Down syndrome. The NT is the measurement of the fluid collection at the back of the fetal neck in the first trimester. A thickened NT may be associated with fetal chromosomal and structural abnormalities as well as a number of genetic syndromes. Patients who desire non-invasive assessment of their risk for aneuploidy can have first trimester screen (a fetal nuchal translucency (NT) measurement and a maternal serum PAPP-A) and a second trimester quadruple screen. The sequential screen which combines the first trimester screen with a quadruple screen yields a 95% detection rate for Down syndrome at a 5% false-positive rate. Since the fetus in this case had a thickened NT, this patient should be scheduled to have a detailed fetal ultrasound and echocardiogram at 18-20 weeks to rule out anomalies. However, it is not possible to diagnose a chromosomal abnormality with an ultrasound. Similarly, although genetic sonograms (targeted sonogram) focus on markers associated with Down syndrome, they are not diagnostic. Approximately 50% of cases of Down syndrome do not have ultrasound findings. Of note, the American Congress of Obstetrics and Gynecology (ACOG) recommends that all patients be offered aneuploidy screening and invasive prenatal diagnosis as indicated.