Multiple pregnancy — chorionicity, surveillance and timing of birth

It is the first-trimester booking scan. A 32-year-old woman who conceived by IVF is told she is carrying twins. The first question she asks is whether they are identical — and the question that matters clinically is whether they share a placenta. Chorionicity determines the entire antenatal pathway: the surveillance schedule, the timing of birth, and the risk profile. Get it right at 12 weeks, and the rest follows; get it wrong, and a monochorionic pregnancy is missed. This topic is the chorionicity-driven framework that holds the whole of multiple-pregnancy care together.^[1]

Overview and definition

Multiple pregnancy is the presence of more than one fetus. Twins are classified on three axes — zygosity, chorionicity and amnionicity — but only chorionicity and amnionicity drive management.^[1][11]

• Zygosity — monozygotic (identical, from one zygote) versus dizygotic (fraternal, from two zygotes). Zygosity is rarely determined antenatally and does not drive management.
• Chorionicity — dichorionic (two placentas) versus monochorionic (one shared placenta). This is the key determinant of risk.
• Amnionicity — diamniotic (two amniotic sacs) versus monoamniotic (one shared sac).

The three clinically important twin types are dichorionic diamniotic (DCDA), monochorionic diamniotic (MCDA) and monochorionic monoamniotic (MCMA). DCDA twins approach near-singleton outcomes; MCDA twins carry substantially higher perinatal mortality because of the shared circulation (twin-twin transfusion syndrome, twin anaemia polycythaemia sequence, selective FGR); MCMA twins carry the highest mortality from cord entanglement.^[1]

References13ShowHide

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