A premature baby weighing 268 grams at the time of delivery left the hospital in good health and weighing more than three pounds after spending five months in the intensive care unit, the Keio clinic in Tokyo said on Wednesday.
Never had a baby born with so little weight left the maternity hospital healthy, according to hospital records.
Doctors decided on premature birth after just 22 weeks of pregnancy as a result of lack of growth.
When he was born, it was in the hands of an adult, doctors recalled.
After five months of intensive care at the neonatology unit, the baby weighs 3,238 kilos.
"I'm happy because he gained so much weight, I honestly did not believe he could survive," her mother said in a message to the media.
The survival rate of newborns weighing less than 300 grams is very small, said the hospital, especially when they are men.
Cases such as this – in which a male baby born less than 300 grams left the hospital in good health – were registered in Germany in 2009 (274 grams), Japan in 2009 (297 g), 2011 (294 g) and 2015 (289 g), according to the international registration of the University of Iowa in the United States.