It's pretty simple really. The brain is primarily consistent of water, of which a large volume of that water is present in the place of blood. When a women is pregnant the body will naturally divert blood away from many other parts of the body in order to feed the child oxygen via the placenta. This includes blood that would usually flow into the brain. As pregnancy goes on and the need for more oxygen by the child is necessary this diversion is even more severe. In other words, the body drains blood from the brain in order to keep the child alive. As for why it takes a while to go back in, the brain is a very spongey tissue, and as such will need to extend back out in order to retake in the same amount of blood it did before.

This you can believe or assume I made up with my basic knowledge of biology from a D grade A-level on the subject. The choice is yours.