The review article discusses abnormalities associated with an abnormal number of autosomes. Abnormalities in chromosome number arise due to non-disjunction. The paired chromosomes do not separate properly. This usually happens during the metaphase of the cell division phase. Hence, it leads to abnormality in the chromosome number. This phenomenon is known as nondisjunction. It occurs either during mitosis or meiosis. There are many causes of non-disjunction. The main cause of nondisjunction is the advanced maternal age. The advancing age of the mother increases the risk of chromosomal abnormalities and nondisjunction in the zygote. There is a direct relation between suspended inactivity of the primary oocyte and the chromosomal abnormalities. Meiosis I in primary oocyte completes at the ovulation phase of the menstrual cycle. Primary oocyte remains suspended until the age of 45, after which it undergoes meiosis II. It happens as the age progresses further. As the mother’s age increases, there arises abnormality in spindle formation. Hence, it may predispose to non-disjunction. Another reason behind nondisjunction is the delayed fertilization after ovulation.