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Multifetal pregnancy Etiology : More commonly fertilization of two ova with tow sperm double ovum or fratermal or dizygotic twins About tow third. A bout one third as often One fertilized ovum divided into two similar structures, into a separate individual that we call it a single ovum – monozygotic or Identical twins Either or both process may be involved in the formation of high number of fetuses . Genesis of monozygotic twins If division occurs within first 72 hours before the inner cell mass is formed and the outer layer of blastocyst is not commited to chorion , They will evolve two embryoes ,with diamniotic ,dichorionic monozygotic. The frecuency 18-36% There may be two distinct placenta or may be a single fused placenta . If division occurs between the fourth and eighth day ,Two amniotic sac will be covered with a common chorion diamniotic , monochorionic ,monozygotic . If the amnion is established ,witch occurs about 8 days fertilization mono amniotic ,mono chorionic, monozygotic twin pregnancy will result . If devision is after this , that is after formation of embryonic disk cleavage is incomplete and conjoined twins are formed .