Most common cause of Downs syndrome is -

Down syndrome is the most common of the chromosomal disorders. About 95% of affected persons have trisomy 21, so their chromosome count is 47 The correlation with maternal age suggests that in most cases the meiotic nondisjunction of chromosome 21 occurs in the ovum. Indeed, in 95% of cases the extra chromosome is of maternal origin. (Robbins Basic Pathology,9th edition,pg no. 237 )