Baby Saybie weighed the same as a large apple when she was born at 23 weeks and three days in December 2018 at Sharp Mary Birch Hospital in San Diego, California.
Fighting for life, she was transferred to the intensive care unit of the hospital and doctors told Saybie's parents she had just hours to live.
But five months on, she was discharged earlier this month weighing a healthy 2.5 kg, confounding all expectations, CNN reported.
On Wednesday, the hospital announced that the baby was born weighing 245 grams, which means she is now believed to be the world's smallest surviving infant.
"After experiencing severe pregnancy complications, Saybie's mother gave birth via emergency cesarean section at 23 weeks, three days gestation in the womb," the hospital said at a news conference on Wednesday.
"Doctors said the preterm birth was necessary after they found that the baby was not gaining weight and her mother's life was at immediate risk."
A nurse who cared for Saybie as she battled for survival said her recovery and release was a "miracle."
According to the Tiniest Babies Registry, a database maintained by the University of Iowa, Saybie weighed seven grams less than the title's previous holder, a baby girl born in Germany in 2015.
"For comparison, at birth, she was roughly the same weight as a large apple or a child's juice box," Trisha Khaleghi, the hospital's Senior Vice President and Chief Executive, said during a news conference.
In a video released by the hospital, the mother described the birth as the "scariest day of my life."
Her birth was so premature, and doctors considered her a "micro-preemie" - a baby born before 28 weeks' gestation. Babies are typically born at 42 weeks.
She was so small she could "fit in the palm of the hands of her care team," the hospital said.