Twinning

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Monozygotic vs. dizygotic twins


Identical twins (monozygotic twins) Fraternal twins (dizygotic twins)
Frequency ⅓ of all twin pregnancies ⅔ of all twin pregnancies
Origin Division of the fertilized oocyte into two embryonic layers Fertilization of two oocytes with two mature spermatozoa
Genetics of the individual Genetically identical Genetically different
Chorionic cavity and amniotic sac Varies (see below) Dichorionic-diamniotic

Special features in the development of monozygotic twins


Type Description Time of division of the zygote Frequency in monozygotic twins
Dichorionic-diamniotic The twins have separate chorionic sacs (separate placentas) and separate amniotic sacs. Within the first 3 days after conception ~20–30%
Monochorionic-diamniotic The twins share a single chorionic sac (the twins share a placenta) but have a separate, individual amniotic sac. Day 4–7 after conception ~70%
Monochorionic-monoamniotic The twins share a single chorionic sac (the twins share a placenta) and a single amniotic sac. Day 8–11 after conception ~1–5%
Monochorionic-monoamniotic (conjoined twins) The twins share the placenta and amniotic sac, and are conjoined. From day 12 after conception onwards <0.1%
take separate cars or share a CAB

  • Splitting 0–4 days: separate chorion and amnion (di-di)
  • Splitting 4–8 days: shared Chorion (mo-di)
  • Splitting 8–12 days: shared chorion and Amnion (mo-mo)
  • Splitting 13+ days: shared chorion, amnion, and Body (mo-mo; conjoined)