Mouse and human embryos, cultured in vitro, undergo a delay in development compared with those grown in vivo. This delay can be caused by suboptimal culture conditions, but possible influences of ovarian stimulation cannot be excluded. The objective of this study was to test the hypothesis that both in vitro and in vivo, preimplantation embryonic development and postimplantation fetal development are impaired in superovulated female mice when compared with naturally cycling controls. A delay in in-vitro blastocyst hatching and in-vivo blastocyst formation (P < 0.03 and P < 0.0001 respectively) and a 40% fetal growth retardation (P < 0.0001) were observed after superovulation in comparison with naturally cycling controls. After transfer to non-stimulated foster mothers, blastocysts from stimulated females had a lower implantation rate (P < 0.005), and developed into fewer living fetuses (P < 0.02), more resorption sites (P < 0.02) and had more pronounced growth retardation (P < 0.0001) when compared with blastocysts from naturally cycling controls. In conclusion, superovulation in the mouse causes a delayed embryonic development in vitro and in vivo, an increased abnormal blastocyst formation, a pronounced fetal growth retardation, and an increased number of resorption sites. If this observation in mice can be extrapolated to humans, it may offer an explanation for the delay in embryonic development and the low birth weight observed after IVF.