The report involves Madeline Mann, born in 1989 weighing 9.9 ounces, then the world record; and 7-year-old Rumaisa Rahman, whose 9.2-ounce birth weight remains the world's tiniest. Rumaisa's birth weight was initially reported as several ounces less, but that figure was based on a different conversion scale.
Two other babies born since 1989 weighed less than Madeline, and a German girl was born last year at her same birth weight.
The report was released online Monday in Pediatrics.
It addresses a question that was hotly debated when Madeline was born 22 years ago, remains hot now -- and still has no answer: "What is the real age of viability? No one knows," said Dr. Stephen Welty, neonatology chief at Baylor College of Medicine and Texas Children's Hospital in Houston.
Muraskas and the report's co-authors say most newborn specialists consider babies born after 25 weeks of pregnancy to be viable -- likely to survive -- and so they should receive medical intervention if necessary to breathe. Younger babies are generally in a "gray zone," where intervention isn't always so clear cut, the report suggests.
In Japan, doctors have lowered that threshold -- the gestational age -- to 22 weeks. Normal pregnancies last about 40 weeks.
Some U.S. doctors will attempt to save babies at 22 weeks, but that is not done routinely, said Dr. Edward Bell, a University of Iowa pediatrics professor.
Bell runs an online registry of the world's tiniest babies, born weighing less than about 14 ounces, or slightly less than 1 pound. Since 1936, 124 have been listed. The registry is compiled from doctors' voluntary reports and so does not represent all survivors.
Bell estimates that about 7,500 U.S. babies are born each year weighing less than 1 pound, and that about 10 percent survive.
A medical report from the doctor who resuscitated the infants at a suburban Chicago hospital is both a success story and a cautionary tale. These two are the exceptions and their remarkable health years later should not raise false hope: Most babies this small do poorly and many do not survive even with advanced medical care.
"These are such extreme cases," said Dr. Jonathan Muraskas of Loyola University Medical Center in Maywood, Ill. They should not be considered "a benchmark" to mean that doctors should try to save all babies so small, he said.
Image: Tiny newborn feet, via Shutterstock.