Boys are more likely than girls to be born prematurely and have a higher risk of death or disability regardless of where they are born in the world, according to six global studies on newborns published Friday in the journal Pediatric Research.

Premature birth accounts for one-third of the world’s 2.9 million newborn deaths. It’s the leading cause of infant mortality in the U.S.

Girls have a biological survival advantage because they mature more rapidly in the womb, and their lungs and other organs may be more developed by the time they are born, according to the studies funded through the Bill & Melinda Gates Foundation, which provide the first systematic estimates of disability in preterm babies.

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Perhaps most groundbreaking, the studies show boys are 14 percent more likely than girls to be born preterm, before 37 weeks of pregnancy. That could be because women pregnant with boys are more likely to have placental problems, pre-eclampsia and high blood pressure, which increase the risk of premature births, according to the global studies by nearly 50 researchers at 35 institutions around the world.

“The death rate among boys is something we had inklings of before,” said Joy Lawn, a neonatologist and epidemiologist at the London School of Hygiene & Tropical Medicine and team leader of the new research.

“But we attributed that to girls developing earlier,” Lawn said in a phone interview from London. “We didn’t know boys were more likely to be born preterm. This research puts together data sets around the world showing the biological risk.”

Researchers next need to dig deeper into why baby boys born prematurely are at greater risk of disability or death than baby girls, she said.

“The thing we need to think about differently is how we address prevention,” Lawn said.

Premature babies can have serious health issues as newborns, as well as lasting disabilities ranging from learning problems and blindness to deafness and motor problems, including cerebral palsy.

Newborn conditions, and especially premature birth, are responsible for nearly 10 percent of the global burden of disease for all ages and all countries, according to the March of Dimes.

The gestational age of U.S. preemies is improving, allowing more time for babies to mature in the womb, but the rate of preterm births in the U.S. is still higher than in other industrialized nations.

In the U.S., more women give birth in their late 30s and early 40s, become pregnant with twins or triplets and smoke or have pre-existing medical conditions associated with obesity — all of which increase the risk for premature delivery, Lawn said.

The U.S. Caesarean delivery rate also contributes, she said. In 2011, the Caesarean delivery rate for babies less than 34 weeks of gestation was 46.8 percent and it was 26.3 percent for babies 34 to 36 weeks, according to a report this summer by the federal Centers for Disease Control and Prevention.

Risks for disabilities or impairments are affected by where a baby is born, which researchers already knew but had not documented to the degree contained in the six global studies:

• In upper-income countries, more than 80 percent of babies born under 37 weeks survive and thrive. Risk of death and disability is greatest for those born at less than 28 weeks.

• In middle-income countries, great progress has been made in reducing deaths, the studies found, but the risk of disability for babies born at 28-32 weeks in middle-income countries is double that of upper-income countries.

• In low-income countries, preterm babies are 10 times more likely to die than babies born in upper-income countries. Death is twice as likely as disability in these countries.

“Babies really are being born into different worlds,” Lawn said. “That’s something that needs to be changed.”

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