This is a very interesting piece from Therese Borchard at the Beyond Blue blog on Beliefnet on what the research community is learning about depression and how it differs from long-held beliefs that depression is simply a chemical imabalance. You should read this, and the article in Psychology Today magazine that it points to.
For all of us who are really struggling to understand why we suffer postpartum depression or anxiety, this is really fascinating and enlightening, even though it doesn't offer all the answers. The brain is a complicated and wondrous thing.