The developmental viability of the preimplantation embryo requires the successful formation of a cluster of pluripotent stem cells called the inner cell mass. Development is variably compromised by a range of exogenous stressors (including their production by assisted reproductive technologies). Inbred C57BL/6 strain embryos are particularly susceptible to the stresses associated with embryo culture, whereas hybrid embryos are more resistant, and this is accounted for in part by the overexpression of transformation-related protein 53 in cultured inbred embryos compared with similarly treated hybrid embryos or embryos not subjected to culture. We show here that this loss of viability is a consequence of the Trp53-dependent reduction in the capacity of blastocysts to form a proliferating inner cell mass. Formation of the trophectodermal line was not adversely affected by these stresses.