A method for detecting a chromosomal aneuploidy in a foetus carried by a pregnant female, said method comprising the steps: (a) providing a sample that has been taken from said pregnant female, which sample comprises DNA that originates from cells of a foetus and/or the placenta of a foetus in admixture with differently methylated DNA of maternal origin; (b) treating the DNA present in said sample with a reagent that differentially modifies methylated and non-methylated DNA; (c) determining an amount of a first target species of DNA, being the chromosome relevant to the chromosomal aneuploidy, in said sample by detecting in said sample the presence of methylation at two or more target differentially methylated regions (DMRs) located on said chromosome, said target DMRs differently methylated between DNA that originates from cells of a foetus and/or the placenta of a foetus and DNA of maternal origin, the modification of DNA of the target DMRs by said reagent is sensitive to methylation of DNA, wherein a detected amount of methylated DNA at one or more of said target DMRs indicates said amount of first target species of DNA in said sample; (d) determining an amount of reference species of DNA, being one or more reference chromosomes, in said sample by detecting in said sample the presence of methylation at two or more reference DMRs located on said reference chromosome(s), said reference DMRs differently methylated between DNA that originates from cells of a foetus and/or the placenta of a foetus and DNA of maternal origin, the modification of DNA of such reference DMRs by said reagent is sensitive to methylation of DNA, wherein a detected amount of methylated DNA at one or more of said reference DMRs indicates said amount of reference species of DNA in said sample; and (e) determining relative amount(s), preferably ratio(s), of an amount determined from step (c) and an amount determined from step (d), wherein one or more of said relative amount(s) indicates the presence or absence of the chromosomal aneuploidy in the foetus, wherein, said detections in step (c) and step (d) are made using the same aliquot of DNA of said sample, and in the same vessel, and effectively simultaneously for said target DMRs and said reference DMRs, and using: (x) the same detectable labels(s) for at least two of said reference DMRs; and (y) a different detectable label(s) for at least two of said target DMRs, optionally, wherein in step (e) said relative amount(s) or ratio(s) are compared with threshold(s) and/or reference distribution(s), wherein one or more of said relative amount(s) or ratio(s) higher or lower than said threshold(s) and/or reference distribution(s) indicates the presence of the chromosomal aneuploidy in the foetus. (a) providing a sample that has been taken from said pregnant female, which sample comprises DNA that originates from cells of a foetus and/or the placenta of a foetus in admixture with differently methylated DNA of maternal origin; (b) treating the DNA present in said sample with a reagent that differentially modifies methylated and non-methylated DNA; (c) determining an amount of a first target species of DNA, being the chromosome relevant to the chromosomal aneuploidy, in said sample by detecting in said sample the presence of methylation at two or more target differentially methylated regions (DMRs) located on said chromosome, said target DMRs differently methylated between DNA that originates from cells of a foetus and/or the placenta of a foetus and DNA of maternal origin, the modification of DNA of the target DMRs by said reagent is sensitive to methylation of DNA, wherein a detected amount of methylated DNA at one or more of said target DMRs indicates said amount of first target species of DNA in said sample; (d) determining an amount of reference species of DNA, being one or more reference chromosomes, in said sample by detecting in said sample the presence of methylation at two or more reference DMRs located on said reference chromosome(s), said reference DMRs differently methylated between DNA that originates from cells of a foetus and/or the placenta of a foetus and DNA of maternal origin, the modification of DNA of such reference DMRs by said reagent is sensitive to methylation of DNA, wherein a detected amount of methylated DNA at one or more of said reference DMRs indicates said amount of reference species of DNA in said sample; and (e) determining relative amount(s), preferably ratio(s), of an amount determined from step (c) and an amount determined from step (d), wherein one or more of said relative amount(s) indicates the presence or absence of the chromosomal aneuploidy in the foetus,