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about 29 per 1,000 American births produce twins

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Identical twins:

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*Are always the same sex and blood type.
*Do not always look alike, but sometimes look like mirror images of each other. One child may be right-handed while the other is left-handed.
*Occur in about 3 to 4 out of 1,000 births worldwide. Race and heredity do not seem to affect the occurrence of identical twins

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Fraternal twins:

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*Tend to run in families.
*Are born most frequently to women who are white or of African descent.
*Can be of different sexes and have different blood types.
*May look very different from one another, with different-colored hair and eyes; they may also look alike, as siblings tend to do.

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Natural causes. You have an increased risk for having a multiple pregnancy if you:

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*Are in your thirties. Naturally occurring fraternal twins are most common among women aged 35 to 40.
*Are black, non-Hispanic, particularly between age 35 and 45.
*Are white, non-Hispanic.
*Have already had a multiple pregnancy. Women who have carried fraternal twins have double the usual risk of having twins.
*Have a family history of fraternal twins or triplets. (A history of multiple pregnancy on your partner's side of the family does not increase your risk of multiple pregnancy.)
*Become pregnant in the first menstrual cycle after stopping birth control pills.
*Have been taking opiate drugs (such as morphine or heroin).

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http://my.webmd.com/hw/being_pregnant/hw236274.asp?lastselectedguid={5FE84E90-BC77-4056-A91C-9531713CA348}

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http://my.webmd.com/content/pages/2/3608_753.htm?lastselectedguid={5FE84E90-BC77-4056-A91C-9531713CA348}