A newborn girl weighing 8.6 ounces (245g) – about the same as an apple – is believed to be the world's tiniest surviving baby.

A San Diego hospital revealed the birth of the girl, who was born in December, and said she is believed to be the world's tiniest surviving micro-preemie.

The girl was born 23 weeks and three days into her mother's 40-week pregnancy.

Doctors told her father after the birth that he would have about an hour with his daughter before she died.

Sharp HealthCare/AP

"But that hour turned into two hours, which turned into a day, which turned into a week," the mother said in a video released by Sharp Mary Birch Hospital for Women & Newborns.

More than five months have passed, and she has gone home as a healthy infant, weighing five pounds (2kg).

The baby's family gave permission to share the story but wanted to stay anonymous, the hospital said. They allowed the girl to go by the name that nurses called her: "Saybie."

Her ranking as the world's smallest baby ever to survive is according to the Tiniest Baby Registry maintained by the University of Iowa.

Dr. Edward Bell, a professor of pediatrics at the University of Iowa, said Saybie had the lowest medically confirmed birth weight submitted to the registry.

But "we cannot rule out even smaller infants who have not been reported to the Registry," he said in an email to The Associated Press.

The hospital said the girl officially weighed 7g less than the previous tiniest baby, who was born in Germany in 2015.

In the video produced by the hospital, the mother described the birth as the scariest day of her life.

World's Smallest Baby Born in San Diego at Sharp Mary Birch Hospital youtu.be

She said she was taken to the hospital after not feeling well and was told she had preeclampsia, a serious condition that causes high blood pressure, and that the baby needed to be delivered quickly.

"I kept telling them she's not going to survive, she's only 23 weeks," the mother said.

But she did. The tiny girl slowly gained weight in the neonatal intensive care unit.

A pink sign by her crib read "Tiny but Mighty."

"You could barely see her in the bed she was so tiny," nurse Emma Wiest said in the video.

Nurses put a tiny graduation cap on her when she left the unit.

The girl faces enormous challenges as a micro-preemie, who is an infant born before 28 weeks of gestation. Micro-preemies can experience vision and hearing problems, developmental issues and a host of other complications.

Many do not survive the first year, said Michelle Kling of the March of Dimes, a non-profit that works to improve the health of mothers and babies.

So far Saybie has beaten the odds.

"She's a miracle, that's for sure," said Kim Norby, another nurse featured in the video.

Twitter felt the same.

Enjoy the rest of your life, sweet Saybie.