There are reports by Diamond and her colleagues, based on measurements made shortly after parturition, that pregnancy increases the size of the cerebral cortex in rats. As a prelude to examining the cellular basis for this phenomenon, we measured the size of the cortex in three coronal planes in hooded rats on the last day of their pregnancy and in control, nonpregnant female littermates. No differences were found between the pregnant and control rats. We conclude that either the events on the day of parturition alter the size of the cortex or the effects of pregnancy on the cortex are not reliable.
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