1. A method for prenatal screening for trisomy 21, comprising obtaining a two or three dimensional image of a fetal face, measuring the angle between a line extending along the upper aspect of the maxilla and a line extending from the anterior aspect of the maxilla to the external surface of the frontal bone at the point of its greatest anterior excursion (the FMF bone angle) on the image, and comparing the measured FMF bone angle with an FMF bone angle characteristic of chromosomally normal fetuses, 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.