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This article has been peer-reviewed. It is the author's final version prior to publication in the Journal of Diagnostic Medical Sonography, Jan/Feb 2006 issue, Vol. 22, No.1. The published version is available at Copyright is retained by Sage Publications.


The determination of chorionicity and amnionicity in multiple gestations is one that confounds many in the medical field. The importance of diagnosing the type of multiple gestation cannot be overstated. This is intended to be an introductory primer and refresher to chorionicity and amnionicity. We will first examine what determines mono- versus dichorionic twinning including zygosity and cleavage of the zygote.