First-Trimester Combined Screening — Marker Pattern (T21/18/13)

The first-trimester combined test (Nicolaides/Fetal Medicine Foundation methodology, used in national programmes such as NHS FASP) multiplies a maternal age- and gestation-specific background risk by likelihood ratios derived from nuchal translucency (NT) and the maternal serum markers PAPP-A and free beta-hCG, producing a numeric risk such as 1-in-N for trisomy 21, 18 and 13. That calculation depends on the Wright et al. (2008) NT mixture model and laboratory- and population-specific marker medians, all implemented in licensed, regularly-recalibrated software (certified FMF/Astraia systems or an accredited laboratory's own combined-test engine) — reproducing those exact coefficients outside that certified software risks generating a confident-looking but inaccurate percentage, which this app will not do. What is robust and well established in the literature is the qualitative direction each marker moves in each condition: trisomy 21 is classically associated with an increased NT, low PAPP-A (commonly cited as below about 0.4 MoM), and a raised free beta-hCG (commonly above about 2.0 MoM); trisomies 18 and 13 are classically associated with an increased NT alongside LOW values of both PAPP-A and free beta-hCG, distinguishing them from the trisomy 21 pattern; an NT of 3.5 mm or more (independent of crown-rump length) is treated as markedly increased in most protocols and prompts fetal echocardiography and detailed anomaly scanning regardless of the biochemical pattern, because increased NT is also associated with cardiac defects, other genetic syndromes, and structural anomalies beyond aneuploidy alone. This tool reports that pattern-level interpretation only. It never outputs a percentage or 1-in-N risk, and a reassuring pattern here does not exclude an elevated risk on certified combined-test or NIPT screening, which should be used for any patient-facing risk estimate.