First-trimester combined screening for Down syndrome (trisomy 21) includes maternal age, nuchal translucency (NT), free β-hCG, and PAPP-A. In a trisomy 21 pregnancy at 12 weeks, the expected pattern of serum markers is:

• A NT increased, PAPP-A increased, free β-hCG decreased
• B NT normal, PAPP-A decreased, free β-hCG increased
• C NT increased, PAPP-A increased, free β-hCG increased
• D NT increased, PAPP-A decreased, free β-hCG increased ✓

Explanation

In trisomy 21 (Down syndrome) at 11–13+6 weeks, the characteristic first-trimester screening pattern is: increased nuchal translucency (due to impaired cardiac function and altered lymphatic drainage), decreased PAPP-A (placental insufficiency, impaired trophoblastic production), and increased free β-hCG (compensatory upregulation). This combination achieves an 85–90% detection rate for trisomy 21 at a 5% false-positive rate in the first-trimester combined test (FMF protocol).

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.