Prenatal Screening, First-Trimester Screening


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Subject: Prenatal Screening, First-Trimester Screening

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  • Performed between 11 and 13 weeks of gestation, first-trimester screening combines maternal age plus two serum biochemical markers: pregnancy-associated plasma protein A (PAPP-A) and β-hCG. It also includes fetal nuchal translucency (NT) measurement.


  • Risk assessment for trisomy 21


  • Increased NT associated with trisomy 13, trisomy 18, trisomy 21, 45,X, triploidy, and other chromosome aberrations.

  • Trisomy 21 biochemical profile typically has increased β-hCG and decreased PAPP-A.

  • Trisomy 18 has decreased β-hCG and decreased PAPP-A.

  • Combining NT and maternal serum profile detects approximately 85% of affected trisomy 21 pregnancies with a 5% positive screening rate.


  • Does not detect neural tube defects.

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

  • NT measurement requires experienced ultrasonographers.