Nuclear dimensions have been proven to be an important indicator of cellular health and nuclear shape-changes are often associated with pathological processes. However, there is still little known about the typical development of nuclear morphology over early mammalian embryonic stages. Here, we have investigated changes in the shape and size of mouse embryo nuclei from 8 cell stage to the developmental day E5.5. We concluded that nuclear size generally decreases until the 128-cell stage, before increasing at E5.5, and the nuclei also follow a general trend of increasing ellipticity over this period. We also found that there were significant differences in nuclear dimensions between nuclei of different lineages from the 32-cell stage onwards, opening the possibility to use nuclear shape and size as non-invasive assessment of lineage formation in mammalian embryos.