The aim of this study was to determine whether a relationship exists between decrease in fecundity and
structural changes in the antimesometrial endometrium of the mouse. Fecundity was calculated as the
number of animals showing a placental sign/number of copulated animals ×100 (%). Structural changes in
the endometrium were examined by electron microscopy. A negative correlation between age and fecundity
was found. Fecundity was 50% at 7 mo of age. At this age, amorphous material appeared in the region
between the basement membrane deep to the luminal epithelium and the subepithelial cells. This material
was sometimes attached to the basement membrane. It increased in amount with advancing age, as fecundity
decreased. The structure of the uterine luminal epithelial cells did not alter with age. The results indicated
that decrease in fecundity with advancing age is correlated with the appearance of amorphous material
beneath the basal lamina of the endometrial epithelium. It is suggested that this could impair
communication between the luminal epithelium and the endometrial stroma, which plays an important role
in implantation.