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One of two infants born sharing some common anatomical parts.
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TWINS A. Monozygotic twins with one placenta, one chorion, and two amnions. B. Dizygotic twins with two placentas, two chorions, and two amnions; SEE: fetus papyraceus; SEE: Hellin's law

Per 1000 live births, incidence rates for American whites are 1:88; for American blacks, 1:70. Generally, the rates are higher in blacks and East Indians and lower in Northern Europeans.

Identical and fraternal twins provide a unique resource for investigating the origin and natural history of various diseases and discovering the different rates of environmental and hereditary factors in causing physical and mental disorders. Esp. important are studies that follow the course of identical twins separated shortly after birth and who then grew up in different social, economic, educational, and environmental conditions. In other research, the second-born twin was found to be at increased risk for an unfavorable outcome, e.g., need for intubation and resuscitation, lower 5-min Apgar score, even when delivered by cesarean section.

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