1. A method for prenatal screening for trisomy 21 using an ultrasound imaging system comprising an ultrasound device and a computing device, the method comprising: obtaining a two or three dimensional image of a fetal face using the ultrasound device, measuring a FMF bone angle on the image using the computing device, wherein the FMF bone angle is the angle between a line extending along an upper aspect of a maxilla and a line extending from an anterior aspect of the maxilla to an external surface of a frontal bone at its point of greatest anterior excursion (the FMF bone angle), and comparing the measured FMF bone angle with an FMF bone angle characteristic of chromosomally normal fetuses using the computing device to provide an indication of an occurrence of trisomy 21 in a fetus, wherein a measured FMF bone angle greater than the FMF bone angle characteristic of chromosomally normal fetuses provides an indication of an increased likelihood of the occurrence of trisomy 21 in the fetus.