It’s not unusual that the newborn found dumped in a Sydney stormwater drain was able to survive days without fluid or food, experts say.

Sydney obstetrician and gynaecologist Dr Andrew Pesce said babies have evolved to be able to live for days without feeding.

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“If it had been a really sickly child at birth it probably wouldn’t have been able to survive,” he said.

“If it has underlying good health, they’re evolved to be able to withstand malnutrition in the first few days.”

For most mothers, milk will “come in” – increase in quantity and begin the change from colostrum to mature milk – between days two and five, said Dr Pesce, the former president of the Australian Medical Association.

“It’s quite normal for a baby to lose up to 10 per cent of its birth weight in the first few days after birth because of this.

“That’s why a baby – that would otherwise seem to be very vulnerable – is actually quite robust in a situation where it’s not getting fluid or foods.”

If the baby boy was dropped down a 2.5 metre drain, a combination of naturally soft bones and the blanket would have protected him from serious injury, he said.

“I’m not saying they can’t get other injuries, to their liver or spleen, but the fact it was wrapped in a blanket probably protected it to an extent,” he said.

Associate Professor Kei Lui, of the University of NSW, said the baby was extremely lucky that the weather was warm in recent days.

“Hypothermia is probably the biggest killer of abandoned babies,” he said.

“It was extremely lucky in this situation.

“We’ve had quite warm days so the temperature has been maintained – it hasn’t been too cold or too hot.”

In addition, leaves or rubbish may have served as a buffer to the baby’s fall, he said.

“If it’s true the baby was dropped from 2.5 metres – it needed to have a soft landing for it to survive.”