1. A method for detecting the presence or absence of a chromosomal abnormality in a fetus, the method comprising: (a) performing a multiplex amplification for at least 70 loci in a single reaction, wherein the reaction comprises cell free DNA derived from maternal blood, and wherein the amplification comprises ligating oligonucleotides that hybridize to target sequences and amplifying the ligated oligonucleotides using PCR; (b) performing microarray analysis to measure an amount of genetic material at the at least 70 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; (c) determining a probability of the presence and a probability of the absence of a chromosomal abnormality in the fetus by comparing the amount from step (b) to either (i) a threshold value or (ii) an expected amount for a particular copy number hypothesis; (d) identifying the presence or absence of a chromosomal abnormality in the fetus by selecting the probability most likely to be true; and (e) 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.