Stepping outside the completely understandable anger and outrage that always accompanies threads like this -
I always maintain that it is pointless looking for rational reasons for this woman's appaling treatment of her child, because she is a long way from being a rational person.
Due to her upbringing, she is utterly devoid of the normal maternal instincts and moral compass that guide almost every parent.
Those inbuilt urges to protect and care are simply not there, rather than ignored, which is what rational thought would indicate - falsely in my view.
I would guess that she spent her time away in an alcohol / drug induced stupor, which, in tandem with her own stunted and absent notions of caring adult behaviour, probably meant that she simply did not think about her child for the entire time - with the tragic consequences that resulted.
People like this have no control over their lives, they live an existance where things simply happen to them, mostly bad things, while they lack the ability to take control of their own lives, far less the lives of unfortunate dependents.
Hopefully, in prison, there may be a chance for this woman to learn to address the situations she has been in, and in future, to avoid making similar mistakes again.
Simply raging about the horror of what she has done - or more accurately, not done, which is look after her child - is not going to prevent a recurrence when the same circumtances create the same scenario when she is released.
Hopefully this horrible circle of abuse and neglect can be broken - we can only hope.