At just 420 grams - little more than the size and weight of a can of Coke - she was so small her parents Leanne and Scot had been told to instead hope her twin brother Liam, at just 620 grams, could be saved.

But as they braced for the devastating news they had lost their baby girl, a miracle happened. She started to cry.

Shannon - the smallest baby ever born at the hospital - was so weak she had to be given a transfusion of just one-and-a-half teaspoons of blood.

Small enough to fit in the palm of her father's hand, and with a footprint no longer than a 50c coin, what makes Shannon's story even more remarkable is that experts believed she had stopped growing at 22 weeks gestation when blood from the placenta started to reverse. The generally accepted minimum age that a baby can survive outside its mother's body is 24 weeks.

What followed after birth was a harrowing wait for her family, as they watched her weight drop further to just 386 grams before slowly increasing out of the danger zone.

''Very intense, beautiful and certainly humbling,'' is how mum Leanne described the experience to the Sunday Canberra Times.

Since their birth in May, both Shannon and Liam have so far shown no signs of serious complications as a result of their early arrival and are now a more healthy 2.2 kilograms and 3.3 kilograms respectively.

''You spend your time willing them through,'' Leanne said.

The parents' ordeal began when doctors presented the couple with a terrible choice. They could either resuscitate Shannon and risk a host of potentially devastating complications that could leave her seriously disabled, or they could elect not to resuscitate their daughter. Fortunately, it was a decision they never had to make when she started breathing on her own.

The oxygen masks used on regular premature babies were too small for Shannon so a Canberra Hospital nurse fashioned a special one to ventilate the tiny girl.

Nurses scrounged for an oxygen pipe small enough for Shannon's throat and eventually found a tube which had an opening just two millimetres in diameter.

She has now been in the world for 12 weeks but would have been born in the past three weeks if she had stayed in her mum's womb for a full nine months.

While the immediate danger has subsided, it will be a three-year wait before their parents find out if either child is clear of any lifelong complications resulting from their early arrivals.

But while Shannon's story seems like a miracle, many others are not so fortunate.

Over recent decades doctors have continually pushed back the cut-off point where premature babies can be saved. But it is an area fraught with risk.

Between the 24th and 29th week in the womb, the baby doubles its weight while connections between nerve cells and fibre ends in the brain are formed - the basis for consciousness in later adulthood.

During this time the retina develops, lungs grow, nostrils open and muscles begin working in preparation for breathing in the outside world.

The most premature baby ever to have survived is believed to be Amillia Taylor, at 283 grams in the United States in 2006, at 21 weeks and six days gestation.

Officially the lightest baby ever born was Rumaisa Rahman at 260 grams in the US at almost 26 weeks.

Those coming too soon risk a host of complications including blindness, deafness, general learning difficulties or cerebral palsy severe enough to hamper them walking without aids. As many as one in five premature babies can be affected.

About 320 of the 700 babies delivered every year at Canberra Hospital are premature. A high proportion because the hospital hosts the region's neonatal unit - usually only 1 per cent of the population's babies end up in this sort of care.

Around 80 of these premature babies are regarded as extremely early arrivals, at less than 32 weeks gestation.

Canberra Hospital Neonatal Intensive Care Unit nurse Sarah Stratford said shock was regularly experienced by parents of premature babies.

''A lot of people don't think they're going to have a premature baby,'' she said.

Parents of babies born on or soon after the risky 24-week mark must decide whether or not they will resuscitate their baby after birth, if the baby does not start breathing on their own after arrival.

This decision is usually made quickly but can have lifelong ramifications for the parents, according to a leading neonatologist at Canberra Hospital, Associate Professor Abdel-Latif Mohamed.

''[The parents] may have to look after a person for the rest of their lives,'' Dr Mohamed said.

Of all the infants delivered between 23 to 31 week gestation, 3 per cent die soon after delivery.

The majority of these are between 23 to 25 weeks gestation.

Some of these children die because parents have decided not to revive the child because of the likelihood of ongoing neurological problems.

A further 10 per cent of babies die after admission to neonatal intensive care.

For mum Leanne Manunui, the past 120 days since her children were born have been an emotional rollercoaster which she was prepared to ride simply because her children have survived.

''It brings things into perspective. If you're having a bad day, all you need to do is think about what your kids have gone through,'' she said.