1. A method of identifying an abnormal fetal chromosome copy number by analysis of a blood sample comprising a mixture of fetal and maternal genomic DNA, the method comprising: a) obtaining a blood sample comprising a mixture of fetal and maternal genomic DNA; b) isolating the mixture of fetal and maternal genomic DNA from the blood sample; c) distributing the isolated mixture of fetal and maternal DNA obtained in step b) into a plurality of at least 500 discrete reaction samples at discrete locations, to randomly provide individual reaction samples that contain a target sequence from a target chromosome and individual reaction samples that do not contain a target sequence from a target chromosome; d) at each discrete location, amplifying genomic DNA with multiple primers to multiple target sequences; and e) counting the number of amplified target sequences representing a fetal chromosome which may be of an abnormal copy number and the number of amplified target sequences representing a fetal chromosome of presumably normal copy number to detect a significantly different number of amplified target sequences representing a fetal chromosome which may be of abnormal copy number compared to the number of amplified target sequences representing a fetal chromosome of presumably normal, thereby identifying an abnormal fetal chromosome copy number.