As a new (and old) dad, this sort of things upsets me so much. As others have said the mother was clearly suffering mentally one way or other and I ask the question where was her support. It is easy to say throw the book at her, but where were her friends and family during this time? The care system? A neighbour? Anyone?
Not sure how you can conclude she was suffering mentally from that report. Her sister said she went off the rails when she was 14 but at 18 she's old enough to know don't leave a baby on their own. The baby was 20 months old so it could be that social services had been visiting but by that time were happy everything was ok!