Monozygotic

Monozygotic twins or identical twins are twins that develop from one zygote, which splits and forms two embryos. This means that they share 100% of their genetic material and are always the same sex. Monozygotic twinning occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide.

Formation

The exact cause of the splitting of a zygote or embryo is unknown. Monozygotic twins can form at different stages of early pregnancy. The timing of the split can affect the later development and individuality of each twin.

Types of Monozygotic Twins

Depending on when the zygote splits, monozygotic twins can be classified into three types: Dichorionic-Diamniotic (Di-Di), Monochorionic-Diamniotic (Mo-Di), and Monochorionic-Monoamniotic (Mo-Mo).

Dichorionic-Diamniotic (Di-Di)

If the zygote splits within the first two days after fertilization, the twins will have separate placentas and separate amniotic sacs. They are known as Dichorionic-Diamniotic (Di-Di) twins.

Monochorionic-Diamniotic (Mo-Di)

If the zygote splits between the third and the eighth day, the twins will share a placenta but have separate amniotic sacs. They are known as Monochorionic-Diamniotic (Mo-Di) twins.

Monochorionic-Monoamniotic (Mo-Mo)

If the zygote splits after the eighth day, the twins will share both the placenta and the amniotic sac. They are known as Monochorionic-Monoamniotic (Mo-Mo) twins.

Genetics

Monozygotic twins are genetically identical (unless there has been a mutation during development) and they are always the same sex. They share the same genotype, but they do not necessarily share the same phenotype, due in part to environmental factors.

See Also

• Dizygotic twins
• Twin
• Zygote
• Genotype
• Phenotype

Monozygotic Resources
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