First-trimester screening. Maternal serum can be analyzed for certain biochemical markers that, in combination with ultrasound measurement of the fetal nuchal translucency (NT), can be used to calculate a risk assessment for trisomies 18 and 21. In the first trimester, these serum markers are the free β-human chorionic gonadotropin (hCG) and pregnancy-associated plasma protein A (PAPP-A). Free β-hCG is elevated and PAPP-A is decreased in a pregnancy affected by Down syndrome. First-trimester screening is an effective screening tool, with a detection rate of 82–87% for trisomy 21 with a 5% false-positive screen rate. First-trimester screening is performed between 10 4/7 and 13 6/7 weeks' gestation and requires confirmation of a chromosomal abnormality by an invasive genetic test. The results are available to the patient early enough that chorionic villus sampling (CVS) is an option for diagnostic testing.
Second-trimester screening. For patients who present after 13 6/7 weeks or choose not to undergo first-trimester screening, the quadruple screen test (Quad screen) is an option. The Quad screen yields a risk assessment for trisomies 18 and 21; unlike first-trimester screening, though, it also provides a risk assessment for open neural tube defects. It involves analyzing levels of maternal serum alpha fetoprotein (MSAFP), total hCG, unconjugated estriol, and inhibin A between 15 and 21 weeks' gestation. In a pregnancy affected by Down syndrome, both MSAFP and unconjugated estriol are low; hCG and inhibin A are elevated. The Quad screen has a detection rate of 81% for Down syndrome at a 5% false-positive screen rate. Like first-trimester screening, the Quad screen requires an invasive test to confirm the diagnosis of a chromosomal abnormality (ie, amniocentesis).
For those patients who chose to undergo first-trimester screening and/or a CVS, neural tube defect screening in the form of a second-trimester MSAFP level should be offered. This maternal serum analyte is elevated in the presence of an open neural tube defect. Evidence exists that focused ultrasound during the second trimester is an effective tool for detecting an open neural tube defect.
Sequential, contingent sequential, and integrated screening. These options involve a combination of first- and second-trimester screening. Specifically with the integrated screen, an ultrasound measurement of the fetal NT is performed between 10 4/7 and 13 6/7 weeks. In addition, maternal serum levels of PAPP-A are obtained in the first trimester and the Quad screen obtained in the second trimester. The results are reported when all the tests are complete. In sequential screening, NT, PAPP-A, and free β-hCG are measured in the first trimester followed by a Quad screen in the second trimester. The results are reported to the patient after completion of the first-trimester portion of the test and then again after the second-trimester portion. Although this test has a high sensitivity, it has a high false-positive rate because it involves two independent tests. The contingent sequential screen does the first-trimester portion of the sequential testing and follows with the Quad screen when an elevated risk is noted. The contingent sequential screening has an ...
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