The smallest baby boy to have survived birth has left a Japanese hospital months after he entered the world weighing just 268g.

The infant was discharged from Tokyo’s Keio University Hospital at a weight of 3.2kg, following six months in intensive care.

Doctors feared for the boy’s life after he was born through an emergency caesarean last August, having failed to gain weight at 24 weeks’ gestation. He was so tiny he could be held in the palm of an adult’s hand.

“I am grateful that he has grown this big because, honestly, I wasn’t sure he could survive,” the infant’s mother said.

The boy was allowed home last week, two months after his initial due date, and is now able to be breastfed, the hospital said on Wednesday. Doctors in the neonatal intensive care unit had previously managed his breathing and nutrition.

World's smallest baby pictures Show all 4 1 /4 World's smallest baby pictures World's smallest baby pictures Five days after his birth in August 2018, weighing 268 grams Reuters World's smallest baby pictures A more recent, undated photo of the baby boy Reuters World's smallest baby pictures Baby boy on February 27 2019, at 3.2kg AFP/Getty World's smallest baby pictures Five days after his birth in August 2018, weighing 268 grams Reuters

The infant, who has not been named, is only the fourth male baby weighing less than 300g known to have survived birth. Nineteen girls weighing less than 300g have survived.

The previous smallest boy was born in Germany in 2009, weighing 274g, according to the University of Iowa’s Tiniest Babies Registry.

‘I wasn’t sure he could survive,’ says boy’s mother (Reuters) (REUTERS)

Babies born below 1kg are likely to suffer heart failure, breathing problems or contract infectious diseases due to their underdeveloped organs.

The survival rate of such babies in Japan has been as high as 90 per cent in recent years, according to The Japan Times, but the rate plunges dramatically for infants weighing less than 300g.