Monozygotic twin pregnancy

Last revised by Dr Jeremy Jones on 20 Sep 2021

A monozygotic (MZ) twin pregnancy results from the division of single zygote following fertilisation and shares similar genetic materials. These twins are therefore always of the same gender.

Monozygotic twins account for approximately 30% of all twin pregnancies 1. The estimated incidence is at ~1 in 250 births with little racial variation for this subtype 3 (cf. dizygotic twin pregnancy).

Depending on the time of division of the zygote, there can be many possibilities

Prenatal diagnosis of chorionicity is important as monochorionic pregnancies have increased rates and severity of all types of obstetric complications when compared with dichorionic pregnancies. Monozygotic twins have a higher rate of fetal anomalies although they tend to be discordant-only affecting one twin despite identical genetic makeup.

Monochorionic twin pregnancies share the one placenta and are therefore prone to hemodynamic complications such as:

Monochorionic monoamniotic twins also carry additional cord related complications such as:

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