Systems, methods, and apparatuses for performing a prenatal diagnosis of a sequence imbalance are provided. A shift (e.g. to a smaller size distribution) can signify an imbalance in certain circumstances. For example, a size distribution of fragments of nucleic acids from an at-risk chromosome can be used to determine a fetal chromosomal aneuploidy. A size ranking of different chromosomes can be used to determine changes of a rank of an at-risk chromosome from an expected ranking. Also, a difference between a statistical size value for one chromosome can be compared to a statistical size value of another chromosome to identify a significant shift in size. A genotype and haplotype of the fetus may also be determined using a size distribution to determine whether a sequence imbalance occurs in a maternal sample relative to a genotypes or haplotype of the mother, thereby providing a genotype or haplotype of the fetus.