Monozygotic Twins Monozygotic (identical) twins one oocyte fertilized by one sperm (twins genetically identical) zygote forms a blastocyst in which the inner cell mass divides into two 2 different amniotic sacs but share common chorion and placenta Conjoined twins one oocyte fertilized by one sperm (twins identical and joined by tissue bridge) formed similar to monozygotic twins but inner cell mass splits in two incompletely 1 amniotic sac, 1 chorion, 1 placenta Dizygotic Twins Fraternal twins two oocytes each fertilized by different sperm (twins genetically different) no more identical than two siblings born at different times 2 amniotic sacs, 2 chorions, 2 placentas