Performed during the second trimester (~16-18 weeks) of pregnancy, the triple marker screen includes maternal serum alpha fetoprotein (msAFP), unconjugated estriol (uE3), and human chorionic gonadotropin (hCG). The markers are relative predictors of risk for abnormalities like neural tube defects and Down syndrome. Combined with maternal age and ultrasound dating of gestational age, the detection rate can be as high as 75%, with a 5% false positive rate.

As a medical student, it's important to remember some general trends. For example, hCG is usually elevated in Down syndrome, while msAFP and uE3 are decreased or normal. All three are decreased in Trisomy 18 (Edwards syndrome). For neural tube defects, msAFP is usually elevated. And the markers are not useful for detecting Trisomy 13 (Patau syndrome).

A quadruple marker test is also available, which adds inhibin A to the triple marker screen, and results in a higher detection rate (81%) for Down syndrome (inhibin A is increased).