The trisomies are chromosomal anomalies which usually occur due to non-disjunction. The vast majority of affected fetuses are spontaneously aborted, often very early during gestation. Only three are compatible with extrauterine life (T13, T18, T21), and only one beyond early infancy (T21).
In order of decreasing frequency they are:
- trisomy 21: Down syndrome
- trisomy 18: Edward syndrome
- trisomy 13: Patau syndrome
Individuals with other rare trisomies may survive beyond infancy, but only if they have mosaic forms.
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