There was a New Scientist article in this the other day.
In late pregnancy it's true that mothers brains shrink (by 7% I think) but this recovers by 6 months post birth and I think it's not associated with any measurable cognitive decline.
In fact, they haven't been able to show any cognitive decline in pregnancy, due to pregnancy.
In fact, the NS article was mainly about cognitive and emotional improvements in the months and years after birth.
But the sleep deprivation, which is largely a societal issue relating to the expectations and support (or lack of) for new mothers, of course has a huge cognitive effect.
I'll find the link later...