WASHINGTON -- Boys are slightly more likely to be born premature than girls, and they tend to fare worse, too, says a new report on the health of the world's newborns.

"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 per cent 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 per cent 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 per cent 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 per cent 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.

For the public, the gender difference may be the most surprising finding of Friday's report, although Dr. Edward McCabe of the March of Dimes says pediatric specialists have long noticed that baby boys start out a bit more vulnerable.

"People are curious about it. We'd like to understand why this occurs," McCabe said.

One possible reason: Mothers have a higher risk of certain pregnancy complications -- high blood pressure and placenta abnormalities -- when carrying boys, Lawn said.

And if a boy preemie and a girl preemie are born at the same gestational age, the boy will be at higher risk of death or disability, she said. But Friday's report concluded there is too little information to quantify how big that risk is.

"Girls walk sooner than boys. They talk sooner than boys. They develop more quickly. That's also true in utero," Lawn said. For a preterm baby, "the difference of a few days maturity between a boy and a girl can mean the difference between major lung complications or not."

It's not just a prematurity issue. The report found that full-term boys also are more likely than girls to experience other newborn health problems including birth complications.

Other findings from the studies, funded by the Bill & Melinda Gates Foundation and published in the journal Pediatric Research:

  • In high-income countries -- the U.S., Canada, Australia and most of Europe -- more than 80 per cent of preemies both survive and thrive, although babies born even a little premature are more likely than full-term babies to be rehospitalized or have learning and behavioural challenges.
  • In low-income countries, preterm babies are 10 times as likely to die as those in high-income countries, and death is more common than surviving with a serious disability.
  • Aside from preterm birth, other leading causes of death and disability among newborns include birth complications that block breathing; severe jaundice and infections.