A computer implemented method for analyzing a biological sample of a pregnant female to determine whether at least two fetuses of the pregnant female are dizygotic, the biological sample comprising fetal and maternal DNA, the method comprising: creating a histogram by: for each of a plurality of chromosomal regions: identifying one or more loci in the respective chromosomal region at which a respective first allele and a respective second allele are detected in the biological sample; measuring, at the one or more loci, a first amount of the one or more first alleles and/or a second amount of the one or more second alleles in the biological sample; and obtaining a normalized parameter for the first amount or the second amount; and incrementing counters based on a number of chromosomal regions with specified values for the normalized parameter; identifying chromosomal regions corresponding to loci at which the mother is homozygous and at least one of the fetuses is heterozygous or corresponding to loci at which the mother is heterozygous and at least one of the fetuses is homozygous; fitting a multi-component mixture model to the histogram corresponding to the identified chromosomal regions, the multi-component mixture model including a mixture coefficient for each of a plurality of components; and determining whether at least two of the fetuses are dizygotic using at least two of the mixture coefficients, wherein at least two of the fetuses are determined to be dizygotic when at least two of the mixture coefficients are above a threshold. creating a histogram by: for each of a plurality of chromosomal regions: identifying one or more loci in the respective chromosomal region at which a respective first allele and a respective second allele are detected in the biological sample; measuring, at the one or more loci, a first amount of the one or more first alleles and/or a second amount of the one or more second alleles in the biological sample; and obtaining a normalized parameter for the first amount or the second amount; and incrementing counters based on a number of chromosomal regions with specified values for the normalized parameter; for each of a plurality of chromosomal regions: identifying one or more loci in the respective chromosomal region at which a respective first allele and a respective second allele are detected in the biological sample; measuring, at the one or more loci, a first amount of the one or more first alleles and/or a second amount of the one or more second alleles in the biological sample; and obtaining a normalized parameter for the first amount or the second amount; and identifying one or more loci in the respective chromosomal region at which a respective first allele and a respective second allele are detected in the biological sample; measuring, at the one or more loci, a first amount of the one or more first alleles and/or a second amount of the one or more second alleles in the biological sample; and obtaining a normalized parameter for the first amount or the second amount; and incrementing counters based on a number of chromosomal regions with specified values for the normalized parameter; identifying chromosomal regions corresponding to loci at which the mother is homozygous and at least one of the fetuses is heterozygous or corresponding to loci at which the mother is heterozygous and at least one of the fetuses is homozygous; fitting a multi-component mixture model to the histogram corresponding to the identified chromosomal regions, the multi-component mixture model including a mixture coefficient for each of a plurality of components; and determining whether at least two of the fetuses are dizygotic using at least two of the mixture coefficients, wherein at least two of the fetuses are determined to be dizygotic when at least two of the mixture coefficients are above a threshold.