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Introduction

A twin pregnancy occurs either when two oocytes are fertilized by two different sperm (dizygotic twinning) or from the division of a single zygote after fertilization of a single oocyte (monozygotic twinning).  Dizygotic twins have different genetic profiles (ie, fraternal twins) and may be the same or different sex.  Monozygotic twins are of the same sex, are genetically identical, and are similar in appearance (ie, identical twins).

Twin gestations are more commonly subdivided by chorionicity (number of placentas) and amnionicity (number of amniotic sacs) ( Figure 1).  This distinction is important due to the increased pregnancy risks when placentas or amniotic sacs are shared.

  • "Diamniotic twins" is a term referring to a pregnancy in which each twin has its own amniotic sac.  Diamniotic twins can be further separated into:

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