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Identical Twins from WikiPedia:

Twins are two offspring resulting from the same pregnancy, usually born in close succession. They can be the same or different sex. Twins can either be monozygotic (MZ, colloquially "identical") or dizygotic (DZ, colloquially "fraternal" or "non-identical").

The general term for more than one offspring in the same pregnancy (multiple birth) is multiples; a fetus which develops alone in the womb is called a singleton.

The number of living human twins in the world has been estimated to be approximately 125 million in 2006 (roughly 1.9% of the world population), with just 10 million monozygotic twins (roughly 0.2% of the world population and 8% of all twins). The twin birth rate in the United States in 2004, 2005 and 2006 was slightly above 32 twin live births per 1,000 live births.

Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting only 37 weeks (3 weeks less than full term) on average. Since premature births can have health consequences for the babies, twin births are often handled with special precautions.

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