The most accurate method of identifying Trisomy 21 during pregnancy is:

• A. First trimester NT measurement combined with PAPP-A
• B. Quad screen results from the second trimester combined with NT and PAPP-A from the first trimester
• C. Quad screen results from the second trimester alone
• D. Quad screen results from the first trimester with NT and PAPP-A

Answer: (B)

Combining multiple screening signals across pregnancy gives the strongest ability to identify Trisomy 21 noninvasively. The first-trimester assessment uses nuchal translucency with maternal serum markers like PAPP-A (and often free beta-hCG) to estimate risk early on. The second-trimester quad screen adds markers such as AFP, hCG, estriol, and inhibin A, which together provide substantial discriminatory power. When you bring the data from both trimesters together, you improve detection while keeping false positives reasonable, making this approach more accurate than relying on either trimesters alone.

The other options rely on a single time-point or misapply the screening pattern (for example, a quad screen is a second-trimester test, so using it in the first trimester isn’t standard). So integrating second-trimester quad screen results with first-trimester NT and PAPP‑A markers yields the highest overall accuracy for identifying Trisomy 21 noninvasively.