Have you read up at all about postpartum psychosis? From what I understand, it can be vicious. And that's what an insanity defense is all about. Someone under a psychosis such as this won't see themselves as damaged, it is up to family and friends to identify it.
I believe that the plea in a case like this should be "guilty by reason of insanity", and that psychological treatment should be in order, not punishment.
Oh, we have some older threads on this: using Google's search, they are:
Andrea Yates to get new trial - Tilted Forum Project
Texas mom's murder convictions overturned - Tilted Forum Project