Will Dunham, Washington: About a quarter of babies born in vaginal deliveries had a small amount of bleeding in their brains, while none delivered by Caesarean section did, according to a study published on Tuesday.

But the researchers at the University of North Carolina School of Medicine in Chapel Hill said it was premature to view their surprising findings as an endorsement of C-sections. They said the findings suggest brain bleeding in some newborns has been common place in vaginal deliveries throughout history, but is being detected now only because of highly sophisticated imaging technology.

"There's no evidence that these bleeds are associated with problems in later life in either mental or physical function or ability," said Dr. Honor Wolfe, an obstetrician involved in the study published in the journal Radiology.

Pressure on the baby's skull while squeezing through the birth canal probably causes the bleeding, said Dr. John Gilmore, a psychiatry professor involved in the study.

The study involved a relatively small number of babies. The researchers used high-resolution magnetic resonance imaging on 88 newborns, with equal numbers of boys and girls, an average of two weeks after birth. Of them, 65 were delivered vaginally and 23 by C-section, in which the baby is surgically delivered through the mother's abdomen.

Seventeen babies delivered vaginally - 26% - had bleeding in and around the brain, called intracranial hemorrhages. Most was located in the lower, rear part of the brain. Not one C-section baby had bleeding.

"While the vast majority of these are probably normal and go away and don't cause any problems, some of them were bigger and, we don't know, but may cause problems down the line as well," Gilmore said.

Small bleeds usually heal harmlessly, but larger ones could lead later in life to learning or motor development problems or seizures.

Gilmore said the researchers will see the children again at ages 1 and 2 to see if they have any enduring problems. Wolfe said no studies have found C-sections offer long-term advantages to children, adding they are less safe for the mother than vaginal deliveries.

C-sections are favored when doctors think vaginal delivery could cause medical complications, but have become increasingly common for what would be considered normal births.

30% of U.S. births in 2005 were by C-section. The researchers said the bleeding was unrelated to the baby's size or head circumference, the length of labor or the use of forceps or vacuum during delivery.

Bones in a newborn's skull are not yet fused, allowing them to shift and overlap as the baby exits the birth canal - a process that can compress the brain or tear blood vessels, causing bleeding, Gilmore said.Most were small amounts of bleeding between the brain and the membrane that covers it inside the skull, called subdural hematomas.

A previous British study found 10% of babies in vaginal deliveries had brain bleeding. The sharpness of the imaging in the new study may account for the higher rate, said Dr. Keith Smith, a radiologist in the study.