While pregnant, it is important to consume Iron, and Cod Liver oil or OMEGA 3 fats. If your baby doesn't get enough of these nutrients, at birth, the baby will pull it out from the mother's brain! Hence, postpartum depression.
Another helpful tip I wanted to share with you all is to give as much oxygen to your fetus as possible. This will help/allow the fetus to keep a lot of its brain cells. There's a lot to it than just that, but you can research the rest...
I hope this was helpful.