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Subject: Prenatal Diagnostic Screening
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Noninvasive testing with goal of limiting invasive diagnostic procedures that carry risk to pregnancy.
Screening modalities have been developed for Down syndrome/trisomy 21 detection because trisomy 21 is the most common viable autosomal chromosome abnormality. However, screening also provides specific risk assessment for trisomy 18 and neural tube defects.
In addition, with inclusion of early ultrasound examination, increased fetal nuchal translucency may indicate other chromosome abnormalities including Turner syndrome (45,X), trisomy 13, and triploidy.
Second-trimester measurement of AFP is used to assess risk of fetal neural tube defects.
Screening is offered to all women, regardless of age, to provide more accurate risk information than provided by age alone.
Risk for trisomy 13 is not calculated; however, trisomy 13 pregnancies are typically associated with ultrasound anomalies detectable with second- trimester ultrasound examination. By definition, screening is not diagnostic; most screen-positive pregnancies are chromosomally normal, and some affected pregnancies will be missed.