"This is a double whammy for boys," said Dr. Joy Lawn of the London School of Hygiene & Tropical Medicine, who led the team of researchers. "It's a pattern that happens all over the world."
The gender difference isn't large: About 55 percent of preterm births in 2010 were male, the report found. Nor is it clear exactly why it happens.
The finding comes from a series of international studies being published Friday that examine newborn health and prematurity. About 15 million babies worldwide are born too soon, most of them in Africa and parts of Asia where survival is difficult for fragile newborns. Globally, about 1 million babies die as a direct result of preterm birth and another million die of conditions for which prematurity is an added risk, the researchers calculated.
Friday's report offers some of the first estimates of how many preemie survivors go on to suffer certain disabilities, and found that where these babies are born, and how early, determines their risk.
Overall, Lawn said about 7 percent of survivors have two of the most burdensome disabilities: neurologic-developmental impairment ranging from learning disabilities to cerebral palsy, and vision loss.
But the biggest risk is to the youngest preemies, those born before 28 weeks gestation. Worldwide, 52 percent of them are estimated to have some degree of neurodevelopmental impairment, the report found.
Moreover, the risk of impairment in middle-income countries is double that of wealthy countries like the U.S.
For example, China is saving more preemies' lives but at the cost of their vision, Lawn said.
Middle-income countries are missing out on a lesson the U.S. learned the hard way several decades ago, that giving these tiny babies too much oxygen can trigger a potentially blinding condition called retinopathy of prematurity.
"Disability is not something that's inevitable. It's preventable," she said, calling for improved quality of care including eye checks to prevent or reduce vision loss.
The March of Dimes reported this month that 11.5 percent of U.S. births now are preterm. That rate is inching down, thanks mostly to fewer babies being born just a few weeks early as standards for elective deliveries have tightened, but it still is higher than in similar countries.
Image: Newborn baby, via Shutterstock