Definition of Twin

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Twin: One of two children produced in the same pregnancy and born during the same birth process.

Twins can develop from one ovum (egg) or from two ova (eggs):

• Twins from one ovum: Twins that develop from a single ovum are called monozygotic. They are said to identical twins and have identical genomes.
• Twins from two ova: Twins that develop from two ova that are fertilized at the same time are called dizygotic. They are nonidentical twins and have different genomes.

The term "fraternal" is often applied to nonidentical twins but it may be misleading. Nonidentical twins can be of the same sex (two brothers or two sisters) or of different sex (brother and sister). The better term is "nonidentical."