Screening for Down's syndrome. The role of intact hCG and free subunit measurement.

The role of intact, total, free alpha and free beta hCG in Down's syndrome screening is reviewed. I discuss the evidence for the enhanced detection of Down's syndrome in the second trimester using hCG beta and review the data with respect to first trimester screening with hCG subunits. I conclude that Down's syndrome screening using hCG is now well established and that increasing evidence from both retrospective and prospective studies now supports the improved detection achievable using assays that specifically measure hCG beta.