Prenatal Diagnostic Screening


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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.