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Boys are more likely to be born preterm, stillbirth or suffer neonatal death. Researchers believe that understanding why girls are more likely to survive during infancy may help significantly lower preterm birth rates.

The latest study revealed that male and female babies develop in radically different ways, and the placenta plays an essential role in these gender differences.

"Our research has found that there are undeniable genetic and physiological differences between boys and girls that extend beyond just the development of their sexual characteristics," senior author of the paper Professor Claire Roberts, leader of the fetal growth research priority for the Robinson Research Institute, said in a news release.

"We've known for some time that girls are clearly winning in the battle for survival, with markedly better outcomes for female babies for preterm birth, stillbirth, neonatal death, and other complications after birth, such as macrosomia (a baby that weighs more than 4-4.5kg or 8 pounds 13 ounces at birth). Male babies generally grow faster and bigger than females. This occurs in both the animal and human worlds, but until now we haven't really understood how or why," said Roberts.

After examining 300 placenta samples, researchers found that more than140 genes were expressed differently across male and female samples.

"Our results suggest that there is a distinct sex bias in the regulation of genes in the human placenta," lead author Sam Buckberry of the University of Adelaide said in a news release.

"We found that with female babies, there is much higher expression of genes involved in placental development, the maintenance of pregnancy and maternal immune tolerance," Buckberry added.

"This suggests that girls are more likely to adopt a risk-averse strategy towards development and survival, and it goes some way to explaining the differences in male and female development in the womb," he said.

"These findings may be important to help guide future sex-specific therapeutics for pregnant women and for babies in the neonatal nursery," Roberts concluded.

The findings were published in the journal Molecular Human Reproduction.