Mice in utero while their mother is lactating suffer higher frequency of deficient corpus callosum.

Mice of the inbred strain BALB/cCF which were conceived during the post-partum estrus and developed in utero while their mother was nursing a previous litter had twice the frequency of agenesis or deficiency of corpus callosum as their close relatives which did not overlap with another litter. There was no harmful effect of overlapping litters on body or whole brain size, so the phenomenon must have resulted from some aspect of maternal environment other than quantity of nutrition.