It has been shown that epigenetic abnormalities are involved in both abnormal fetal development, as represented by large offspring syndrome, and the onset of diseases affecting vital prognosis, such as metabolic syndrome and malignancy. And if assisted reproductive technology (ART) causes epigenetic abnormalities, then this must be avoided. We compared the expression of imprinted genes (Igf2, Peg3, Snrpn and Kcnq1ot1), which are expressed in early embryos and are involved in fetal and placental development, using in vivo and in vitro mouse blastocysts. In order to compare the changes in gene expression under different culture conditions, in vitro embryos were incubated using two culture media (KSOM/AA and M16) and two gas phases (O₂ 5% and 20%). Between in vivo embryos and KSOM/AA embryos, no marked differences were seen in gene expression. However, between in vivo embryos and M16 embryos, changes in gene expression patterns were confirmed. By changing the incubator's O₂ concentration, even when using the same culture medium, changes in imprinted gene expression were confirmed, clarifying that the expression of imprinted genes in early mouse embryos is related to culture medium components and oxygen concentration. It is important for ART to improve culture conditions.