The world's smallest baby boy, who was born in October in Japan weighing as much as an apple, is now ready for the outside world, his doctor said Friday.

Ryusuke Sekiya was delivered via emergency Caesarean section, after 24 weeks and five days of pregnancy as his mother Toshiko experienced hypertension.

At 258 grams he was even lighter than the previous record holder, another Japanese boy who weighed just 268 grams when he was born last year. That baby was discharged from a Tokyo hospital in February.

When Ryusuke was born on October 1, 2018, he measured 22 centimetres tall, and medical staff kept him in a neonatal intensive care unit.

They used tubes to feed him, sometimes taking cotton swabs to apply his mother's milk to his mouth.

Nearly seven months later, the boy has grown 13 times in weight, now weighing over three kilogrammes. He will be released from Nagano Children's Hospital in central Japan over the weekend.

When born, the baby weighed as much as an apple. File photo.

"When he was born, he was so small, and it seemed as if he would break with a touch. I was so worried," his mother Toshiko told reporters.

"Now he drinks milk. We can give him a bath. I am happy that I can see him growing," she said. 

The smallest surviving girl was born in Germany in 2015 weighing 252 grams, according to a registry put together by the University of Iowa of the world's tiniest surviving babies.

The survival rate for tiny babies is substantially lower for boys than for girls.