Eight proven and easily implementable measures could prevent more than 565,000 stillbirths in these countries, according to a series of papers published in the Lancet journal.
The measures included providing micronutrient, protein and energy supplements, low-dose aspirin, the hormone progesterone, education on the harms of smoking, and treatments for malaria, syphilis and bacteria in urine.
If steroids were made available to pregnant women and doctors did not immediately clamp the umbilical cord, the deaths of more than 475,000 newborn babies could also be prevented, the research found.
Implementing these changes would cost an estimated $1.1 billion, the researchers said.
This is "a fraction of what other health programs receive," said Per Ashorn, a lead study author and professor at Finland's Tampere University.
Another study author, Joy Lawn of the London School for Hygiene and Tropical Medicine, told AFP that the researchers used a new definition for babies born premature or underweight.
She said the traditional way to determine a baby had a low birthweight - if it was born weighing under 2.5 kilogram- was "a bit randomly selected" by a Finnish doctor in 1919.
The researchers analyzed a database that included 160 million live births from 2000 to 2020 to work out how often babies are born "too soon and too small," she said.
"Quite shockingly, we found that this is much more common once you start to think about it in a more nuanced way."
The researchers estimated that 35.3 million - or one in four - of the babies born worldwide in 2020 were either premature or too small, classifying them under the new term "small vulnerable newborns."
While most of the babies were born in southern Asia and sub-Saharan Africa, Lawn emphasized that every country was affected.