The twin placenta: framework for gross analysis in fetal origins of adult disease initiatives.

Current data strongly support that a wide variety of adult health risks are influenced by intrauterine life events. Twins may have the same (identical, monozygous) or different (fraternal, dizygous) genes, but their relationship to their outside world, via their placentas, is rarely equal. Monochorionic twins and dichorionic twins both resemble and differ from singletons. In the balance between the similarities and the differences, we may find the answer to the interpretation of twin studies, and appropriately apply them to the 'fetal origins' hypothesis.