Fetal maternal samples taken from pregnant women include both maternal cell-free DNA and fetal cell-free DNA. Described herein are methods for determining a chromosomal abnormality of a test chromosome or a portion thereof in a fetus by analyzing a test maternal sample of a woman carrying said fetus, wherein the test maternal sample comprises fetal cell-free DNA and maternal cell-free DNA. The chromosomal abnormality can be, for example, aneuploidy or the presence of a microdeletion. In some embodiments, the chromosomal abnormality is determined by measuring a dosage of the test chromosome or portion thereof in the test maternal sample, measuring a fetal fraction of cell-free DNA in the test maternal sample, and determining an initial value of likelihood that the test chromosome or the portion thereof in the fetal cell-free DNA is abnormal based on the measured dosage, an expected dosage of the test chromosome or portion thereof, and the measured fetal fraction.