Women in their mid-30s experience a marked decline in fertility. The origin of these fertility defects resides in the implantation capacity of the embryo itself, but the mechanistic basis of this impairment is not well understood. Here we identify a core mechanical defect in embryos from aged females that impairs their implantation competence. Using mouse models, we find that reproductive ageing drives excessive contractility in the trophectoderm, the outer epithelial lineage that enables implantation. This hypercontractility increases blastocyst tissue surface tension and viscosity, which hinders spreading during implantation. Elevated contractility is both necessary and sufficient for age-associated implantation failure. We identify non-invasive imaging signatures that infer embryo mechanics and predict implantation success for embryos of both young and aged females. Analyses of human embryos and in vitro fertilization clinical datasets reveal conserved age-associated mechanical alterations that correlate with implantation potential. Our work implicates embryo mechanics as a key regulator of reproductive longevity.