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  1. Aneuploidy: the state of having a chromosome number that is not a multiple of the haploid number.

  2. Combined first trimester screening: computation of risk assessment for fetal aneuploidy using a combination of sonographic evaluation of fetal nuchal translucency with the following maternal serum biochemical markers: pregnancy-associated plasma protein-A (PAPP-A) and either the free β-subunit of, or intact, human chorionic gonadotrophin (fβ-hCG or t-hCG).

  3. Contingent screening: also known as "sequential contingency" screening—a screening strategy that requires second trimester screening only in those women whose first trimester results are reported as "borderline. Subsequent second trimester screening incorporates earlier screening results in risk calculation.

  4. Fully integrated screening: a screening strategy that assigns risk based on the combination of first and second trimester screening results, with nondisclosure of results until the completion of second trimester testing.

  5. Independent sequential screening: the combination of first and second trimester screening results, but with disclosure of first trimester results, such that only patients with a negative first trimester screen are recalled for second trimester testing. The second trimester screening results do not take into account the results of the earlier screen.

  6. Multiple of the median (MoM): the division of a measurement by the median value for the normal population.

  7. Nuchal translucency: hypoechoic area in the fetal neck region, the thickness of which is measured sonographically between 11 and 14 weeks' gestation as a marker for fetal aneuploidy.

  8. Quadruple ("quad") test: second trimester measurement of serum α-fetoprotein (AFP), unconjugated estriol (uE3), human chorionic gonadotrophin (hCG), and inhibin A.

  9. Septated cystic hygroma: enlargement of the hypoechoic space at the back of the fetal neck, extending along the length of the fetal back, with clearly visible septations.

  10. Stepwise sequential screening: a screening strategy whereby patients are offered first trimester results, and subsequently first trimester markers are included in the second trimester risk assessment.

The ability of prenatal ultrasonography to identify fetal abnormalities has in recent times been harnessed toward early detection of aneuploidy and of congenital abnormalities in the fetus. Advances in technical skill and in the resolution of modern-day ultrasonography have facilitated the drive toward prenatal diagnosis at ever-earlier gestations. This move has been fueled by the recognition that prenatal diagnosis early in pregnancy offers invaluable early reassurance to patients with negative results and lower complication rates for those who choose not to continue affected pregnancies.

The combination of sonographic evaluation of fetal nuchal translucency with quantification of maternal serum biochemical markers forms the backbone of first trimester screening for aneuploidy. The objectives of first trimester screening are continually being widened beyond aneuploidy screening to include sonographic surveillance for a range of anatomic defects. Nonetheless, it is critically important to note that pivotal to the success of any screening strategy for fetal abnormality is the accuracy with which the required markers are obtained.

A host of screening paradigms now exists, with ...

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