Quad Screen Risk Estimate

About this calculator

The quad screen risk estimate models the second-trimester prenatal screening test that measures four maternal serum markers — AFP (alpha-fetoprotein), hCG, unconjugated estriol (uE3), and inhibin-A — each compared against expected values for gestational age to produce a likelihood ratio. This calculator multiplies that likelihood ratio by the maternal-age-based baseline risk of trisomy 21 to produce an adjusted risk, the same logic used in the actual test report, typically expressed as an odds ratio like 1 in 250.

Because maternal age alone is a weak predictor (most Down syndrome babies are born to mothers under 35, simply because far more pregnancies occur at younger ages), combining it with biomarker data substantially improves detection. Obstetricians and genetic counselors use quad screen results, usually performed between 15 and 20 weeks, to decide whether further diagnostic testing like amniocentesis is warranted — but as with any screening test, this is a probability, not a diagnosis.