1. A method for determining the number of copies of a chromosome or chromosome segment of interest in the genome of a gestating fetus, the method comprising: obtaining measured genetic data at a plurality of loci on a chromosome or chromosome segment of interest in the a mixed sample of DNA comprising fetal DNA and maternal DNA; determining, on a computer, a ratio of fetal to maternal DNA in the mixed sample from the measured genetic data; creating a plurality of hypotheses specifying the number of copies of the chromosome or chromosome segment of interest in the fetus; determining, on a computer, the probability of each of the hypotheses using the measured genetic data and the ratio of fetal to maternal DNA; and selecting the hypothesis with the greatest probability, thereby determining the number of copies of the chromosome or chromosome segment of interest in the genome of the fetus.