CHICAGO—Baby boys are more likely to die than baby girls and medical advances have actually increased the gender gap, a study released Monday found.

An analysis of infant mortality in 15 developed countries found that baby boys are 24 percent more likely to die than baby girls.

This is down from a peak of 31 percent in 1970, but double the rate in the days before the development of vaccines and public health measures like improved sanitation dramatically improved infant mortality rates.

"During the great historical improvements in infant mortality, the rising male disadvantage in infancy revealed a level of unexpected male vulnerability," the study published in the Proceedings of the National Academy of Sciences concluded.

"As infant mortality falls to very low levels, infant deaths become increasingly concentrated among those who are born with some weakness."

The male disadvantage begins in utero.

Girls have a stronger immune system while boys are 60 percent more likely to be born prematurely and to suffer from respiratory problems, among others. Boys are also more likely to cause risky or difficult labor because of their larger body and head size.

When poor sanitation and nutrition weakened all babies and mothers the male disadvantage was less noticeable: from 1751 until 1870 the gender mortality gap was about 10 to 15 percent.

But the development of the germ theory dramatically cut infectious disease rates, making complications of childbirth and premature birth more common causes of death.

The gender gap rose steadily as infant mortality rates plummeted and only began to reverse with the increased use of caesarean sections and improvements in neonatal care.

Only about five percent of babies born prior to 1970 were delivered by c-section while more than 20 percent of births in the 15 developed nations studied are now performed by c-section.

"Changes in obstetrical practice and neonatal medicine that saved all but the weakest babies have benefited boys more than girls because boys were more vulnerable across the entire range of birth weights," the authors concluded.

"In addition to changes in delivery practices, improvements in neonatal intensive care also may have benefited males more than females."

The 15 countries analyzed include Sweden, France, Denmark, England/Wales, Norway, The Netherlands, Italy, Switzerland, Finland, the United States, Spain, Australia, Canada, Belgium and Japan.