The boy was so tiny at birth that he could fit into a pair of cupped hands and his skin was so thin that his rib cage was clearly visible.
He was treated at a neonatal intensive care unit where doctors managed his breathing and nutrition, and he was eventually able to breast feed.
«I can only say I’m happy that he has grown this big because, honestly, I wasn’t sure he could survive,» the boy’s mother said, according to Keio University Hospital.
Dr Takeshi Arimitsu told the BBC that he wanted to show «there is a possibility that babies will be able to leave the hospital in good health, even though they are born small».
Babies who weigh less than 1.5 kilogram at birth face a range of health problems such as higher risk of infection, trouble breathing because their lungs are so undeveloped, neurological problems such as bleeding on the brain and gastrointestinal problems.
Honestly, I wasn’t sure he could survive.
Baby’s mother, Tokyo
According to a database of the world’s tiniest babies – all weighing less than 400 grams – held by the University of Iowa, the previous record for the smallest boy was a German baby born in 2009, who was known as Tom Thumb by his doctors.
He weighed 275 grams after being born 15 weeks prematurely. He spent nine months in hospital before being allowed home.
The smallest-ever recorded baby was a girl born in Germany in 2015 who weighed just 252 grams. Girls are more likely to survive pre-term birth than boys. Although doctors are still unsure why this is, it could be due to them having better developed lungs.
While this baby boy may seem to have made it through, against the odds, about 50 to 60 per cent of babies born at 24 weeks do survive, said Helen Mactier, a consultant neonatologist in Glasgow.
«Survival is dictated by a number of factors but probably the most important is gestational age. But survival is also dictated by the health of the mother and the complications of pregnancy,» Dr Mactier said.
While babies are still surviving after being born at 23 and even 22 weeks, further improvement was unlikely, she said.
«It’s very hard to see a baby surviving before 22 weeks [because] there are physiological issues which would make that very difficult,» she said.
The Telegraph, London