How Common Are Twins?
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Twin pregnancies are either fraternal or identical. Fraternal twins share the
same womb, but come from different ovum. Genetically, fraternal twins are
as alike as any pair of siblings. Identical twins on the other hand, come
from the same fertilized ovum and theefore share the same genetic makeup.

The ratio of fraternal twin to single births varies from country to country.
The highest incidence occurs in the Yoruba tribe of Nigeria where 46 of 
every 1000 pregancies involves fraternal twins. The lowest is in Japan,
where the incidence of fraternal twinning is 4 per 1000. America and other
European countries have about the same rate of 11 to 12 per 1000 births.

The ratio of identical twin to single births is constant across countries
and nationalities. About 3 out of every 1000 births involves identical twins.
Scientists don't know why some pregancies involve identical twins and 
others do not, but it seems to be independent of such issues as heredity 
and diet.