In mammals, the maternal and paternal genomes are required for embryonic development. This is due to genomic imprinting which leads to the expression or repression of genes solely on the basis of the parent from which they were inherited. As a result, parthenogenetic embryos die before day 10 of gestation and show limited development of extra-embryonic membranes. Maternal imprinting is established during oogenesis and is associated with allele specific modifications in DNA methylation. We have investigated epigenetic modifications during oocyte growth using nuclear transfer techniques to produce mature oocytes with maternal chromatin derived from non-growing oocytes. Parthenogenetic activation of such oocytes leads to the development of normal size fetuses with a well developed placenta on day 13.5 of gestation; three days further than previously recorded for parthenogenetic development. In contrast, after fertilization, only one embryo was recovered on 9.5 days of gestation. Further, in these embryos we investigated the well characterized methylation pattern of the maternally expressed insulin-like growth factor II receptor gene (Igf2r) and found that the pattern of methylation was indeed different to that of fertilized control embryos. Thus, the embryonic phenotypes observed here correlate with changes in epigenetic events that normally occur during oocyte growth.