Zygosity determines the risk of abnormality and chori-onicity what can be done if one is present. Zygosity can be deduced definitively in cases of monochorionicity (=MZ) or discordant external genitalia (=DZ) while in DC concordant-sex twins, the chance of dizygosity is 75-80%. MZ twins have a 50% increase in structural abnormalities per baby, which are sought at the routine anomaly scan at 20 weeks. In particular they have twice the frequency of congenital heart disease, and thus a fourfold increase per pregnancy, so that fetal echocardiography is recommended .
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