1. A method for detecting the presence or absence of a chromosomal abnormality in a fetus, the method comprising: (a) measuring the amount of genetic material at multiple polymorphic loci on a chromosome or chromosome segment of interest in a sample comprising DNA from the fetus and DNA from the mother of the fetus; wherein the amount of genetic material at a particular polymorphic locus is determined irrespective of the identity of the alleles at that polymorphic locus; (b) determining, on a computer, the probability of the presence and the probability of the absence of a chromosomal abnormality in the fetus by comparing the amount from step (a) to either (i) a threshold value or (ii) an expected amount for a particular copy number hypothesis; (c) identifying the presence or absence of a chromosomal abnormality in the fetus by selecting the probability most likely to be true; and (d) outputting the selected probability as an indication of whether the fetus has a chromosomal abnormality.