Next to the absence of significant transgenerational developmental defects, our results showed that CF1 and ICR maternal irradiation with either 0.2 or 0.4 Gy did not lead to a increased genomic instability in the next generation. Indeed, in both strains the level of chromosome damage observed in the cells of control gastrulas was quite comparable to the one found in the cells of gastrulas issued from females irradiated at the zygote stage with either 0.2 or 0.4 Gy. In all groups, chromosome damage was characterized by a predominancy of chromatid-type aberrations over chromosome-type aberrations. In particular, many chromatid gaps were observed, even in the control group. Early post implantation development in mammals is associated with a dramatic increase in the proliferation rate of undifferentiated stem cells that form the primary embryonic layers, ectoderm, endoderm and mesoderm, and with the start of differentiation of the embryo.