Second and first trimester estimation of risk for Down syndrome: implementation and performance in the SAFER study.

Objectives: Document patient choices and screening performance (false positive and detection rates) when three improved Down syndrome screening protocols were introduced coincidentally.

Method: Second-trimester 'triple marker' screening was expanded by adding second-trimester dimeric inhibin-A (four-marker), with or without first-trimester pregnancy-associated plasma protein-A (five-marker). Nuchal translucency (NT) measurements were included when available from accredited sonographers (six-marker).