This is the most detailed, authoritative textbook available on the use of ultrasound markers in screening for fetal chromosomal defects. It describes in detail the methodology for calculating the risks for chromosomal defects using maternal age and gestational age as well as ultrasound findings. The authors explain how more than 90% of major chromosomal defects can be detected by combining data for fetal nuchal translucency thickness, fetal heart rate, maternal age, and maternal serum biochemistry at 10-14 weeks of gestation. The book is heavily referenced throughout and contains many illustrations and tables.