An in vitro method wherein the incomplete or noisy measurement of multiple SNP loci on a given segment of a given chromosome of a target individual is used to determine the number of instances of the given segment in the genome of the target individual, the method comprising: (i) creating a set of one or more hypotheses about the number of instances of the given segment present in the genome of the target individual, (ii) measuring the amount of genetic data for some or all of the possible alleles at a plurality of SNP loci on the given segment, using tools and/or techniques taken from the group comprising Molecular Inversion Probes, Genotyping Microarrays, the Taqman SNP Genotyping Assay, quantitative PCR, the Illumina Genotyping System, other genotyping assays, fluorescent in-situ hybridization (FISH), and combinations thereof; (iii) determining the relative probability of each of the hypotheses given the measurements of the target individual's genetic data and possibly a related individual's genetic data where the determination of the relative probability of each hypothesis is made using quantitative techniques that do not make allele calls, wherein the quantitative techniques that do not make allele calls comprise matched filtering, and where the mean and standard deviation for the measurement of each locus is either known, unknown or uniform; and (iv) using the relative probabilities associated with each hypothesis to determine the number of instances of the chromosomal segment in the genome of the target individual; wherein the determination of the number of the given chromosome segment in the target individual is to screen for a chromosomal abnormality, that abnormality taken from a list comprising monosomy, uniparental disomy, trisomy, other aneuploidies, unbalanced translocation, and combinations thereof and further wherein the target individual is taken from a group consisting of a human fetus, a human embryo, a non-human fetus and a non-human embryo. (i) creating a set of one or more hypotheses about the number of instances of the given segment present in the genome of the target individual, (ii) measuring the amount of genetic data for some or all of the possible alleles at a plurality of SNP loci on the given segment, using tools and/or techniques taken from the group comprising Molecular Inversion Probes, Genotyping Microarrays, the Taqman SNP Genotyping Assay, quantitative PCR, the Illumina Genotyping System, other genotyping assays, fluorescent in-situ hybridization (FISH), and combinations thereof; (iii) determining the relative probability of each of the hypotheses given the measurements of the target individual's genetic data and possibly a related individual's genetic data where the determination of the relative probability of each hypothesis is made using quantitative techniques that do not make allele calls, wherein the quantitative techniques that do not make allele calls comprise matched filtering, and where the mean and standard deviation for the measurement of each locus is either known, unknown or uniform; and (iv) using the relative probabilities associated with each hypothesis to determine the number of instances of the chromosomal segment in the genome of the target individual; wherein the determination of the number of the given chromosome segment in the target individual is to screen for a chromosomal abnormality, that abnormality taken from a list comprising monosomy, uniparental disomy, trisomy, other aneuploidies, unbalanced translocation, and combinations thereof and further wherein the target individual is taken from a group consisting of a human fetus, a human embryo, a non-human fetus and a non-human embryo.