The baby boy before he was taken home from the hospital.

In August 2018, a baby boy was born in Tokyo weighing only 9.45 ounces (268 grams) — about the weight of a bag of potato chips.

The boy had stopped growing in his mother's womb, doctors at Keio University Hospital told Reuters, and had to be delivered by an emergency Caesarean section 24 weeks into the mother's pregnancy to avoid a stillbirth. But the baby was smaller and less developed than he should have been at 24 weeks, which is the youngest age most doctors consider "viable," meaning the child can survive outside the womb. He could not breath or eat on his own, and his entire body fit inside his parents' cupped hands.

But last week, on Feb. 20, after five months of treatment, that baby boy was finally discharged from Keio University Hospital and allowed to go home with his family. Though he now weighs a healthy 7 pounds (3.2 kilograms), he leaves the hospital with the distinction of being the world's smallest surviving male baby born in recorded history. [7 Baby Myths Debunked]

"There are only four babies this small [that survived] that we know of in the history of mankind," said Dr. Edward Bell, a professor of neonatal pediatrics at the University of Iowa's Carver College of Medicine. "It's exceedingly rare. And for boys, as far as I can tell, it's unique."

Bell is the founder and webmaster of the Tiniest Babies Registry, the University of Iowa's database of the world's smallest surviving babies born since 1936. The baby boy born in Tokyo last August ranks as the fourth-smallest child on Bell's registry, and is surpassed by three baby girls from Tokyo, Illinois and Germany, all born at 25 weeks and weighing 9.34, 9.17 and 8.88 ounces (265, 260 and 252 g), respectively.

The baby boy at five days old, in one of his parent's hands. (Image credit: Keio University School of Medicine Department of Pediatrics)

Of the registry's 210 babies (all weighing under 14 ounces, or 400 grams, at birth), Bell told Live Science that roughly 75 percent are female. This is because males develop more slowly than females, both in the womb and in adolescence.

"In these cases of extremely early [birth] or extremely small children, girls are a little more developed than boys," Bell said, and that gives baby girls a slight survival advantage. "More boys are born, but more girls survive."

Often, Bell said, babies born so small stop developing in the womb because their mother's placenta is not properly delivering the oxygen and nutrients they need to grow. The boy from Tokyo had "unusually well developed" heart, lungs, brain and kidneys for a baby of his size, Bell said (the boy's physician had to provide these details to get the baby listed on Bell's registry), but chances are he will never catch up to the average height of other kids.

"Even if his parents are of average size, he is more likely to be one of the shorter kids in his class picture, for example," Bell said.

Babies born so small also face heightened risks of developing learning disabilities and health problems than children born at full term — however, Bell said, the outcomes truly seem to differ baby-to-baby.

"Some of the babies on the registry are severely handicapped," Bell said. "Some of them are having good lives, are college graduates and very talented individuals. You see the whole spectrum."

Originally published on Live Science.