Prenatal Screening, Second-Trimester Screening (Maternal Serum Screening; Quad Screen)

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Subject: Prenatal Screening, Second-Trimester Screening (Maternal Serum Screening; Quad Screen)

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Definition

  • Performed between 15 and 22 weeks of gestation, the quadruple screen combines maternal age plus four serum biochemical markers: hCG, inhibin A, AFP, and unconjugated estriol to assess the risk of trisomy 21 and trisomy 18.

Use

  • Risk assessment for trisomy 21 (Down syndrome), trisomy 18, and open neural tube defects

Interpretation

  • Trisomy 21 profile typically has high levels of hCG and inhibin A with low levels of AFP and unconjugated estriol.

  • Trisomy 18 is associated with low levels of hCG, AFP, and estriol. (Inhibin A does not contribute to trisomy 18 risk profile.)

  • Different centers use different cutoffs, balancing detection rate against number of invasive procedures performed. A cutoff of 1:270 (approximately 5% positive screening rate) detects approximately 80% of trisomy 21 and trisomy 18 pregnancies.

Limitations

  • Detects fewer affected pregnancies than combined first-semester plus second-trimester screening modalities.

  • Does not permit first-trimester decision making regarding termination of affected pregnancies.

 
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