A computer implemented process for the identification of the presence or absence of one or more chromosomal abnormalities in a fetus using a maternal sample comprising both fetal and maternal nucleic acids, the process comprising: i) providinga data set comprising frequency sequence data for a set of at least 16 loci from a single chromosome from two or more maternal samples each comprising both fetal and maternal nucleic acids, wherein each sample is identified by a sample index, and wherein the sequences of the dataset are determined in a single sequencing reaction; ii) normalizing the frequency of a sequence of individual loci within said dataset; wherein locus or loci refers to a nucleic acid region of known location in a genome, and wherein the normalizing comprises: subjecting the detected frequencies of a sequence from an individual locus across all samples to a mathematical transformation based on the frequencies of sequences of the at least 15 other loci across all samples within the data set, to reduce experimentally introduced variation, wherein the mathematical transformation comprises using a regression technique to normalize the frequency of the set of loci; and iii) identifying differences in sequence frequencies of a normalized locus from the fetal chromosome relative to sequence frequencies of loci of one or more other chromosomes in one or more of said maternal samples, thereby identifying the presence or absence of one or more chromosomal abnormalities in a fetus. i) providinga data set comprising frequency sequence data for a set of at least 16 loci from a single chromosome from two or more maternal samples each comprising both fetal and maternal nucleic acids, wherein each sample is identified by a sample index, and wherein the sequences of the dataset are determined in a single sequencing reaction; ii) normalizing the frequency of a sequence of individual loci within said dataset; wherein locus or loci refers to a nucleic acid region of known location in a genome, and wherein the normalizing comprises: subjecting the detected frequencies of a sequence from an individual locus across all samples to a mathematical transformation based on the frequencies of sequences of the at least 15 other loci across all samples within the data set, to reduce experimentally introduced variation, wherein the mathematical transformation comprises using a regression technique to normalize the frequency of the set of loci; and subjecting the detected frequencies of a sequence from an individual locus across all samples to a mathematical transformation based on the frequencies of sequences of the at least 15 other loci across all samples within the data set, to reduce experimentally introduced variation, wherein the mathematical transformation comprises using a regression technique to normalize the frequency of the set of loci; and iii) identifying differences in sequence frequencies of a normalized locus from the fetal chromosome relative to sequence frequencies of loci of one or more other chromosomes in one or more of said maternal samples, thereby identifying the presence or absence of one or more chromosomal abnormalities in a fetus. <EOS>