The baby was born by emergency C-section in August, and was so small he could fit into a pair of cupped hands. The infant was nurtured in intensive care until he was released last week, two months after his due date.

He had grown to a weight of 3.2kg and is now feeding normally. The newborn on the age of 5 days. Born at 24 weeks, the tiny boy spent five months in the hospital.

"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 Tokyo's Keio University Hospital.

Doctor Takeshi Arimitsu, who treated the extraordinary baby, told he was the smallest infant born on record to be discharged from a hospital, according to a database of the world's littlest babies held by the University of Iowa.

The former record-holder used to be a boy born in Germany, weighing 274g. The smallest surviving child lady in that very same database used to be additionally born in Germany, in 2015, and reportedly weighed 252g.

He said he wanted to show that "there is a possibility that babies will be able to leave the hospital in good health, even though they are born small".

Among the very smallest babies, the survival rate is much lower for boys than girls. Medical experts are unsure why, though some believe it could be linked to the slower development of male babies' lungs.