Environmental influences that may precede fertilization: a first examination of the prezygotic hypothesis from maternal age influences on twins.

The prezygotic hypothesis considers the possibility that development is subject to environmental influences on the oocyte prior to conception. Such influences may occur in the maternal grandmother's uterus where oogenesis is completed or in the mother before fertilization. According to this hypothesis, the separate eggs from which DZ twins are derived may be sensitive to microenvironmental variations within an ovary. As a first approach, we examined same-sex MZ and DZ twins for maternal age effects on differences between pairs in cognitive and behavioral traits. While no differences between MZ and DZ pairs were found that would indicate a major effect of the prezygotic environment, suggestions are made for further experimental studies of this unexplored question in human development.