1. A method for detecting the presence or absence of a chromosomal abnormality in a fetus, the method comprising: (a) measuring an amount of genetic material at multiple loci on a chromosome or chromosome segment of interest in a sample comprising DNA derived from the fetus and from the mother of the fetus, wherein the amount of genetic material at a particular locus is determined irrespective of an identity of alleles at that locus; (b) determining, on a computer, a probability of the presence and a probability of the absence of a chromosomal abnormality in the fetus by comparing the amounts 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, thereby detecting the presence or absence of a chromosomal abnormality in the fetus.