Child Development recently put out a monograph (sorry, no link; I'm reading an actual hard copy) on the effects on kids of mom working during the first year of their lives. The subjects were non-Hispanic Whites or African American, in the NICHD Study of Early Child Care.
For kids of moms working full time (but not part time), white kids (but not black kids) had significantly lower scores on some (but not all) cognitive measures at 3, 4.5, and in first grade.
But there were some advantages, too: more money, less stress over money (therefore less cortisol flowing in everyone), and putting the kid in a better preschool when she's 3.
The conclusion to the abstract says, "These results confirm that maternal employment in the 1st year of life may confer both advantages and disadvantages and that for the average non-Hispanic white child those effects balance each other." It's a gratifying conclusion, but I wish they had a comma coach.