Prenatal diagnosis refers to those testing modalities used during pregnancy to screen and diagnose fetal aneuploidies and anomalies.
I. NUCHAL TRANSLUCENCY (NT)
An ultrasound measurement of the amount of fluid behind the neck of the fetus, best done between 10 3/7 and 13 6/7 weeks. An increase in the NT correlates with an elevated risk for chromosomal abnormalities such as trisomy 21 or 18. In addition, those gestations with increased NT have an elevated risk of adverse pregnancy outcomes, including fetal cardiac defects and intrauterine fetal demise, even when karyotype is normal. A measurement of NT alone has a low detection rate for trisomy 18 and 21; therefore, it is not recommended as a sole screening test for aneuploidy.
II. COMBINED FIRST-TRIMESTER SCREENING
Nuchal translucency combined with a measurement of the maternal serum markers, free beta human chorionic gonadotropin (free β-hCG) and pregnancy-associated plasma protein A (PAPP-A), is used to calculate the risk for trisomies 18 and 21. It is performed between 10 3/7 and 13 6/7 weeks' gestation. It is an effective screening tool, with a detection rate of 82–87% for trisomy 21 at a 5% false-positive screen rate. Free β-hCG is elevated and PAPP-A is decreased in a pregnancy affected by Down syndrome.
III. SECOND-TRIMESTER SCREENING
The quadruple screen (Quad screen) involves analyzing levels of maternal serum alpha fetoprotein (MSAFP), total hCG, unconjugated estriol, and inhibin A between 15 and 21 weeks' gestation to calculate the risk for trisomies 18 and 21. In addition, it provides a risk assessment for open neural tube defects. For patients who present after 13 6/7 weeks or choose not to undergo first-trimester screening, the Quad screen is an option. In a pregnancy affected by Down syndrome, both MSAFP and unconjugated estriol are low and hCG and inhibin A are elevated. The Quad screen has a detection rate of 81% for Down syndrome at a 5% false-positive rate. Like first-trimester screening, the Quad screen requires an invasive test to confirm the diagnosis of a chromosomal abnormality (ie, amniocentesis or chorionic villous sampling [CVS]). For those patients who chose to undergo first-trimester screening and/or CVS, neural tube defect screening in the form of a second-trimester MSAFP level should be offered. The MSAFP 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.
IV. INTEGRATED SCREENING AND SEQUENTIAL SCREENING (INDEPENDENT, STEP WISE, AND CONTINGENT)
These options involve a combination of first- and second-trimester screening.
Integrated screen. The NT and maternal serum levels of PAPP-A are obtained in the first trimester, and the Quad screen is obtained in the second trimester. When the test does not include an NT measurement it is called serum ...
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