Mystery mum delivered boy before river find, tests show

A missing mother who might have given birth next to a Sydney river delivered a baby boy, forensic tests have revealed.

Investigators are yet to track down the woman or the child nearly three days after a dog walker found a placenta and umbilical cord on the banks of the Cooks River at Earlwood.

Testing of the material revealed the placenta and cord belonged to a baby boy, police said on Thursday.

Authorities have held fears for the safety of the mother and newborn since the material was found on Monday afternoon by the suburban riverside, which is about 10km southwest of the city centre.

A NSW Police cordon off the scene on the banks of the Cooks River
A dog walker discovered the placenta and cord on the banks of the Cooks River on Monday. (Dean Lewins/AAP PHOTOS)

The mother is being urged to seek medical attention at a nearby hospital as police insist the health of her and her son remains their top priority.

A full-scale search of the area on Tuesday did not show any evidence the baby had been harmed, nor did it reveal any immediate clues to why the placenta and umbilical cord had been left there.

Police divers and a blood detection dog were called in before the search wrapped up later that afternoon.

Tony Robson, who made the initial discovery, said the placenta and umbilical cord were not hidden but simply placed on top of the riverbank.

"It's a secluded dead-end spot … the (dogs) were off-lead having a bit of a romp around and (one dog) jumped down into the mangroves and spotted something," he told Seven News.

"(It was) low tide so it was just sitting on top of the mud, the water was quite a distance away … I've seen three births, you know what you're looking at."

On Tuesday, police identified a potential burial site near the riverbank.

But it turned out to be a false alarm, instead discovering a cat that had been recently buried.

Investigators have suggested the woman might have disposed of the placenta and umbilical cord at the river because she did not want anyone to know about the baby.

"Childbirth, for many people, can be a traumatic time, and it can be very distressing … I am deeply concerned for the safety and the mental health of the mother," Superintendent Christine McDonald said previously.

"There is no judgment, they need to know we are concerned for them."

The placenta and umbilical cord were found not far from a sports field and Islamic centre on Lang Rd, close to a busy footbridge frequented by walkers, joggers and cyclists.

The Cooks River flows from Yagoona in Sydney's outer southwest and runs into Botany Bay at Kyeemagh, 23km away.

Sections are heavily polluted as a result of traffic congestion, litter, sewage, illegal dumping and industrial and domestic activities.

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