Figure 3: Isolikelihood ratio curves for Down’s syndrome screening using the markers beta hCG and PAPP-A. The likelihood ratio (Downs/normals) indicated as 1:64, 1:32 etc. 95% fractal ellipses for normals and for Downs. The arrow indicates the change in log MoM values that would be the result of a ten-day increase in the estimated gestation age.The red arrow shows the change in likelihood ratio for a woman where the gestational age was changed by five days and the blue for a gestational age change of ten days.