The tiny tot – who weighed less than 250g at birth – was discharged from hospital in San Diego on Wednesday.
Baby Saybie was lighter than a hamster when she was born in December 2018.
In a statement, the team at Sharp Mary Birch Hospital for Women and Newborns said the baby now weighed around five pounds.
She is believed to be the world’s smallest surviving newborn baby, according to the Tiniest Babies Register, based at the University of Iowa.
Her unnamed mum gave birth late last year by emergency cesarean section at just 23 weeks pregnant.
At 8.6 ounces, Saybie weighed almost seven grams less than the previous smallest surviving newborn, who was born in Germany in 2015.
The hospital said: “She was small and fragile, and could fit in the palm of the hands of her care team.”
They said the premature birth had been essential because Saybie wasn’t gaining weight, and her mum’s life was at risk.
Babies born before 28 weeks are considered micro preemies.
“She was small and fragile, and could fit in the palm of the hands of her care team”
The hospital said such early births often come with a range of life-threatening medical challenges, but Saybie had escaped largely unscathed.
“Saybie experienced virtually none of the medical challenges typically associated with micro preemies, which can include brain bleeds and lung and heart issues,” it said.
Doctors said the baby’s family had given permission for Saybie’s story to be shared, but wished to remain anonymous.
Last week, Daily Star Online revealed the astonishing story of a Brit baby born without any skin who had made it home after a six-month ordeal in hospital.