Monozygotic Twins (MZ)

Definition - What does Monozygotic Twins (MZ) mean?

Monozygotic (MZ) twins are two babies produced from a single occurrence of fertilization where one sperm and one egg combine to create a single zygote. This zygote divides into two embryos that create separate fetuses.

Monozygotic twins are also known as identical twins.

FertilitySmarts explains Monozygotic Twins (MZ)

Monozygotic twins occur in roughly 3-5 out of every 1000 pregnancies. They are more common in older women. Variations of MZ pregnancies exist depending on when the embryo splits post fertilization. These variations reflect if the fetuses share a placenta (monochorionic) and amniotic sac (monoamniotic), or they have their own placenta (dichorionic) or amniotic sac (diamniotic):

  • 1-3 days - dichorionic-diamniotic twins
  • 4-8 days - monochorionic-diamniotic twins
  • 8-13 days - monochorionic-monoamniotic twins
  • After 13 days - high likelihood of monochorionic-monoamniotic conjoined twins.

The later the embryo splits, the higher the likelihood of conjoined twins.

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