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

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