Octuplets can be fraternal (multizygotic), identical (monozygotic), or a combination of both. Multizygotic octuplets occur from eight unique egg/sperm combinations. Monozygotic multiples are the result of a fertilized egg that splits into two or more embryos.

It is possible for the split to occur more than once, producing monozygotic triplets or potentially even a set of monozygotic octuplets, although none have been documented.

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