A twin is one of two offspring produced in the same pregnancy.Twins can either be identical (monozygotic)meaning that they develop from one zygote that splits and forms two embryos, or fraternal (dizygotic) because they develop from two separate eggs that are fertilized by two separate sperm.
Zygosity is the degree of identity in the genome of twins. There are five common variations of twinning. The three most common variations are all fraternal (dizygotic):

* Male–female twins are the most common result
* Female–female fraternal twins
* Male–male fraternal twins
The other two variations are identical (monozygotic) twins:
* Female–female identical twins
* Male–male identical twins (least common)
The degree of separation of the twins in utero depends on if and when they split into two zygotes. Dizygotic twins were always two zygotes. Monozygotic twins split into two zygotes at some time very early in the pregnancy. The timing of this separation determines the chorionicity and amniocity of the pregnancy.Monoamnionic twins divide after the first week.In very rare cases, twins become conjoined twins.

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