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Twins

There are two types of twins: identical and fraternal (non-identical). Scientists use the terms "monozygotic" and "dizygotic" because these terms are biologically meaningful. Monozygotic twins result from a single fertilized egg that, sometime during the first week following conception, divides into 2 embryonic discs, which develop into 2 separate embryos; dizygotic twins result from 2 eggs being fertilized during the same menstrual cycle.

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