Boys are 14 percent more likely to be born premature than girls, and they are more susceptible to disease and experience disabilities, according to six global studies on newborns to be published Friday in the journal Pediatric Research. Common disabilities include learning disorders and cerebral palsy.

The finding are a part of 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.

However, the gender difference isn't large with only 55 percent male preterm births in 2010.  The reason as to why preterm boys are more susceptible is still unknown.

"One partial explanation for more preterm births among boys is that women pregnant with a boy are more likely to have placental problems, pre-eclampsia, and high blood pressure, all associated with preterm births," said Dr. Joy Lawn of the London School of Hygiene & Tropical Medicine, who led the team of researchers. 

"Baby boys have a higher likelihood of infections, jaundice, birth complications, and congenital conditions, but the biggest risk for baby boys is due to preterm birth," Lawn noted.

The study found 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."

"Even in the womb, girls mature more rapidly than boys, which provides an advantage, because the lungs and other organs are more developed."