I read this in the Self magazine:
"Motherhood:
"Feel like an alien stole your brain? A woman's brain shrinks 8 percent during pregnancy. The whys aren't fully understood,... but the theory is that surges of the bonding hormone oxytocin may be restructuring brain circuits, rerouting energy and nutrients to the regions dedicated to attachment and baby tracking. And never fear: Brain size returns to normal six months post-baby, so if you're till feeling spacey, chalk it up to sleep deprivation and stress. Both have been shown to decrease brain function temporarily in numerous studies."
Explains a lot, doesn't it?
(The source of this information seems to be Louann Brizendine, M.D., a neuropsychiatrist.)