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KEY TERMS

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Key Terms

  1. Screening test: usually non-invasive tests that are designed to identify people with an increased risk for a certain condition before they realize they have it. Nuchal translucency thickness, first trimester screening, quad screening, sequential screening, integrated screening, and maternal cell-free DNA provide risk estimates for trisomy 21 for low risk patients.

  2. Diagnostic test: test aimed at providing certainty about a specific diagnosis in an individual suspected to have the condition. Example: a fetal karyotype obtained by amniocentesis conclusively diagnoses or excludes trisomy 21 in a patient at risk based on screening tests (e.g. nuchal translucency thickness or cell-free DNA screening).

  3. Phenotype: constellation of observable characteristics or traits of an individual.

  4. Karyotype: number and appearance of chromosomes in the cell nucleus displayed as a systematized arrangement of chromosome pairs in descending order of size.

  5. Fluorescent in situ hybridization (FISH): molecular cytogenetic technique that uses fluorescent probes that identify specific DNA sequences on chromosomes. The test provides a quick method to determine the number of copies of a particular chromosome without the need to produce a karyotype.

  6. Chromosomal microarray: method to identify submicroscopic deletions and duplications (copy number variants or CNVs) across the genome via molecular methods.

  7. Variant of uncertain significance (VOUS): allele or variant of a gene whose significance to health is unknown.

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INTRODUCTION

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Congenital anomalies are typically organized by organ system. This is an excellent method that allows similar disorders to be grouped together. However, the multisystem chromosomal anomalies and syndromes do not fit neatly into this organ organizational method. Usually the most striking finding or the most unusual finding is the one that leads to inclusion into one or another group.

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In the following pages, we will review the phenotypic features of several chromosomal anomalies and syndromes that should be familiar to those performing obstetrical ultrasonography. These disorders have been selected either because they are fairly characteristic or common, or because their recognition may have an impact for genetic counseling and pregnancy management. From the outset, we would like to state that this review cannot be exhaustive, as there are entire books (eg, the Birth Defects Encyclopedia with approximately 2000 described syndromes)1 and websites (eg, Online Mendelian Inheritance in Man [OMIM], created in December 1995 and, as of today, beyond the 18,000-syndrome landmark)2 dedicated to this subject.

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Another important aspect of prenatal diagnosis of syndromes and chromosomal anomalies is that a final prenatal diagnosis based on phenotypic features alone is usually not be possible. Indeed, an accurate diagnosis is often achieved only after appropriate prenatal genetic testing and/or after careful examination of the child after birth. The term “appropriate” is used here to emphasize that no single test is currently available that is capable of providing a final diagnosis or of excluding every genetic abnormality in a fetus diagnosed with multiple malformations. Therefore, genetic counselling by a professional who is up-to-date with the ...

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