The 32-year-old’s charges were downgraded from attempted murder to abandoning a child under seven causing it to be in danger of death and recklessly causing grievous bodily harm.
Police allege the boy was dumped in the drain five days before he was found, on November 18.
The mother was arrested after police tracked her down via hospital records and by door knocking in the area.
She will be sentenced at Parramatta District Court on Friday.
The 32-year-old woman, whose baby was found alive after about five days at the bottom of the 2.4-metre drain in 2014, was today sentenced to a maximum three years and six months in jail.
He had lost 20 percent of his birth weight before being admitted to hospital where he was treated for dehydration, a lack of nutrition and given antibiotics.
The court heard the woman had at first seemed happy about the pregnancy but became depressed when her boyfriend told her he had a new girlfriend and that he'd heard someone else had fathered the child.
She was concerned her family would disapprove of the man being Fijian.
The child's current foster carer says he is meeting milestones so far, but he will have to be regularly assessed for long-term effects up to school age.
The woman was given a backdated non-parole period of one year and nine months, meaning she will be eligible for release in August.
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