In the UK two-thirds are in women under 35. Today, maternal serum marker screening is the method of choice. The first marker to be investigated was AFP, as levels were known from neural-tube defect screening. Although AFP is only a weak marker, it led to the development of the concept of 'risk screening' and stimulated the search for other markers. Risk screening uses a statistical model to estimate each individual's risk of Down syndrome from the maternal age and marker profile.
Prenatal diagnosis is offered if the risk exceeds a fixed cut-off.
As well as maximising detection, it provides an understandable figure on which to base counselling. Whilst one of them alone will yield reasonable results, multiple-marker screening is preferable. Statistical models which take account of the shape of the frequency distributions and the correlation between markers are used to predict the discriminatory power of different marker combinations. The predicted detection rate for combinations of hCG and AFP, unconjugated estriol, or both, is about two thirds. Inhibin A is relatively new and it is likely that in the future many will include it as an additional marker, thus increasing the detection rate to about three-quarters.