In a study of 130 ยฎrst trimester cases of trisomy 21 and 959 controls we have shown that the median MoM for alpha-fetoprotein (AFP) is lower (0.82) and that for total human chorionic gonadotrophin (hCG) is higher (1.31) than in the control group. For AFP 15.3% of cases were below the 5th centile and for total hCG 19.8% were above the 95th centile. The median shift observed for AFP and total hCG is poorer than that for pregnancy associated plasma protein-A (PAPP-A) or free b-hCG and together with maternal age, AFP and total hCG could only be expected to detect 40% of cases. In combination with PAPP-A, total hCG would identify 52% of cases, somewhat less than the 67% observed with free b-hCG and PAPP-A. However, we have demonstrated for total hCG a signiยฎcant temporal change in median MoM with gestational age. Before 70 days the median MoM was less than 0.5, between 70 and 83 days this increased to 1.13, and between 84 and 97 days this increased to 1.52. This median shift has signiยฎcant implications for interpreting previous studies and even more signiยฎcant implications for detection rates. When population parameters speciยฎc to the gestational age in question are used, detection rates with total hCG and PAPP-A increase from 47% at 70ยฑ83 days to 60% at 84ยฑ97 days. This observation explains much of the confusion around total hCG in the ยฎrst trimester and shows the importance of selecting analyte pairs and population parameters appropriate to the time in gestation when screening is performed.