At birth, Amillia was only as long as a pen

Amillia at 22 weeks

They had earlier said Amillia Taylor would be allowed to go home on Tuesday.

Amillia is believed to be the first baby to have survived following a gestation period of less than 22 weeks.

She weighed a mere 10 ounces (284 grams) at birth on 24 October. Babies under 14 ounces were thought to stand no chance of survival.

'Miracle baby'

A spokeswoman at the Baptist Children's Hospital gave no details on the doctors' decision, simply saying: "They want to observe her a couple more days."

Doctors held out little hope of Amillia surviving

Amillia spent a little under 22 weeks in her mother's womb, a world record according to the University of Iowa which keeps track of premature babies born throughout the world.

Initially, doctors held little hope for her survival. She measured just 9.5in (241mm).

"She's truly a miracle baby, " said Dr William Smalling, neo-natal expert at the Baptist Children's Hospital.

"We weren't too optimistic. But she proved us all wrong," he said.

Amillia has experienced respiratory problems, a very mild brain haemorrhage and some digestive problems, but doctors now say her "prognosis is excellent".

Even though she's only four pounds (1.8kg) now, she's plump to me

Sonja Taylor,

Amillia's mother

Q&A: Premature babies

Her parents named her Amillia - which means resilient in Latin, a fighter and hardworking - to reflect her survival against the odds.

"It was hard to imagine she would get this far. But now she is beginning to look like a real baby," said Sonja Taylor, Amillia's mother.

"Even though she's only four pounds (1.8kg) now, she's plump to me," Mrs Taylor said.

She said the worst thing had been not being able to hold her baby for more than six weeks after she was born.

On Tuesday, Amillia is going home after spending nearly four months in the hospital's neo-natal intensive care unit where her every move was monitored 24 hours a day.

Doctors say she will still have to be monitored closely after her release, requiring asthma medication and extra oxygen for months to come, as she still weighs less than 4lb 6oz (2kg).

Amillia's survival demonstrates the dramatic advances in neo-natal care in recent years, correspondents say.