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Updated June 25, 2014

Definition: The chorion is the outer membrane of the sac surrounding a fetus in utero. Twins that develop in separate sacs are considered dichorionic. All dizygotic -- or fraternal -- twins are dichorionic. Monozygotic twins may also be dichorionic.
Common Misspellings: dicorionic, dichronic
During a twin pregnancy, it is important to identify the structure of the chorion to determine if it is monochorionic or dichorionic. Dichorionic twins that have separate chorions may be monozygotic or dizygotic.

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