Monozygotic twinning is a relatively rare event in vivo conception, being estimated to occur in 0.42% of all birth. The underlying mechanism for monozygotic twin formation is the division of the embryo early in its development. Separation of cells may theoretically occur before or after inner cell mass formation. The incidence of monozygotic twinning following assisted reproduction techniques is higher than the commonly accepted incidence after in vivo conception. Patients at particular risk of monozygotic twins are those aged > 35 years and those who had manipulation techniques for assisted fertilization. Hence, it seems prudent to c<>unsel these patients about the potential obstetric complications of monozygotic multiple gestations prior to the initiation of their treatment.