A baby boy thought to be the smallest ever to survive has finally been sent home from a Japanese hospital five months after his birth.

The baby weighed just 9.45oz (268g) when he was born 16 weeks prematurely in August 2018.

His life to date has been spent in the neonatal unit at Keio University Hospital, Tokyo, but was finally sent home last week, two months after his due date.

Doctors performed an emergency caesarean section at 24 weeks when the child failed to grow during the pregnancy and experts feared that his life was in danger.

The child, whose name has not been released by hospital authorities, now weighs 7lb (3.2kg) more than 12-times his original birth weight. But the baby is still lighter than the average full-term baby boy in the UK, who would weigh in at 7lb 8oz (3.4kg).

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”.

The world's smallest baby boy is finally allowed home.

The world’s smallest baby boy is finally allowed home.

Babies who weigh less than 3.3lb (1.5kg) 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.

According to a database of the world’s tiniest babies – all weighing less than 14oz (400g) – 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 9.7oz (275g) 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 8.8oz (252g). Girls are more likely to survive preterm 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, around 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,” she 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 as there are physiological issues which would make that very difficult,” she said.

The Telegraph, London