I just think it's an incredibly sad story all round.
I can't even fathom the thinking behind leaving a newborn in a drain. Wether she felt she had no other choice and put the baby somewhere she hoped he would be found or someone forced her to do it, I don't know. I'm glad the baby was found though and hope that he makes a full recovery.
I think people jump straight to mental illness, because they want to believe there is a reason for this, that there is something to blame for this behavior.