If you think PPD (or any other form of depression) is "brain chemistry" on the same level and getting hungry or horny you grossly misunderstand what depression is at a fundamental level.
The serotonin imbalance hypothesis of depression, widespread as it is in popular culture, has actually been pretty thoroughly debunked at this point[1]. We know that SSRIs do have a therapeutic effect, but it seems their effect is oddly enough unrelated to serotonin levels.
What they seem to do is mask the depressive symptoms to the point where you're given a sporting chance to "behave" less depressed, which is what seems to be the most effective treatment. It's fairly analogous to how painkillers can cure back pain by permitting you to move around more.
Not acting like you're depressed is the real cure, which is of course very cruel, because as anyone who has had a depressive episode will attest to, just getting out of bed may be a struggle.
All that being said, if you want spiritual support go to your church, you shouldn't be getting that from your OB or pediatrician.