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 Ž ²-hCG. It also includes fetal nuchal translucency (NT) measurement.
Use
Risk assessment for trisomy 21
Interpretation
Increased NT associated with trisomy 13, trisomy 18, trisomy 21, 45,X, triploidy, and other chromosome aberrations.
Trisomy 21 biochemical profile typically has increased Ž ²-hCG and decreased PAPP-A.
Trisomy 18 has decreased Ž ²-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.
Limitations
Does not detect neural tube defects.
Detects fewer affected pregnancies than combined first-semester plus second-trimester screening modalities.
NT measurement requires experienced ultrasonographers.