First trimester ultrasound assessment for fetal aneuploidy.

Screening tests for trisomy 21 have gradually become more refined and now involve complex statistical models that combine demographic, biophysical, and biochemical parameters to produce individualized risk estimates for pregnant women. An understanding of the evolution of the principles, methods, and statistical techniques applied to Down syndrome screening is valuable as these processes can be transferred to other, more prevalent, adverse pregnancy outcomes. First trimester ultrasound forms the foundation of this process.