Other
Ds Health: Prenatal Screening for Down Syndrome
This essay serves as a guide to prenatal testing for Down Syndrome and describes breakthroughs in ultrasound techniques which allow doctors to detect Down Syndrome without the need for an amniocentesis procedure.
WebMD
Medicine Net: Ultrasound
This site from MedicineNet.com gives an Ultrasound description, why they're performed, risks, and patient preparation. A thoughrough explanation of the process is provided.