Some preterm babies may experience more breathing problems if delivered by cesarean section instead of vaginally, according to new research presented Thursday at the 32nd annual Society for Maternal-Fetal Medicine Meeting.

C-sections have been thought to benefit premature babies and lessen their risk of complications.

But researchers at Johns Hopkins and Yale University who reviewed 2,560 births in New York City from 1995 to 2003 found that underweight premature infants had a 30 percent greater chance of developing breathing problems when delivered by C-section compared to those delivered vaginally.

Dr. Diane Ashton, deputy medical director of the March of Dimes, said the results "overturn conventional wisdom that c-sections have few or no risks for the baby."

One in 8 babies is born prematurely in the United States, according to the March of Dimes.

The findings were limited to a small subgroup of babies who grow in the fetus weighing less than 90 percent of babies at the same gestational age - a condition called intrauterine growth restriction.

The seemingly rare condition can develop if an expectant mother has severe hypertension or kidney disease, or if a baby is malnourished, according to Dr. Lucky Jain, neonatologist and medical director of the Emory Children's Center at Emory University.

In the study, the babies were delivered before 34 weeks of pregnancy.

Pre-term babies may be at higher risk for stillbirth during vaginal delivery, so they are more often delivered by c-section.

While the findings may give physicians pause to consider an alternate delivery method, Jain said the findings are not likely to change practices for most.

"The study tells me that it may not be a bad idea to let the mother give birth vaginally," said Jain, adding that, oftentimes, the decision is made based on an individual mother's situation.

"To change practice, one would have to do a randomized trial, which would be hard to do given the small numbers," said Jain.