Raleigh, N.C. ? Delivering babies by Caesarean section just a few days too early increases the risk of respiratory problems and other complications in newborns, doctors at the University of North Carolina-Chapel Hill and other centers report today.

The findings, published in the New England Journal of Medicine, add to concerns about the rising rate of C-section births in the United States. Since the mid-1990s, women have increasingly elected to schedule surgery instead of undergoing a vaginal birth, so that C-sections now account for more than 30 percent of deliveries in the United States.

Much of the increase stems from women having repeat C-sections for their second and third babies — a choice doctors advocate to avoid a rare but potentially fatal complication when the uterus ruptures at the original incision site. But first-time Caesarean deliveries are also climbing, sometimes just for the convenience of doctor and patient, and other times for medical reasons, such as a stalled labor or a baby positioned feet-first in the womb.

In any case, the timing of the operation is important, the study finds. Scheduling a delivery at 37 weeks of gestation results in a nearly fourfold increased risk of respiratory distress syndrome, in which the baby’s lungs aren’t capable of filling with oxygen, compared with babies born at 39 weeks. Newborns delivered earlier than 39 weeks also were significantly more likely to have other breathing problems and an infection called sepsis.

“I think there is something inherent in labor — probably a signal from baby to mother that the baby is ready to make the transition — that we choose to circumvent when we schedule an elective, repeat C-section,” said Dr. John Thorp, a UNC-CH obstetrician and one of the authors of the study. “I think it points to a role for the baby in the initiation of labor and the baby’s readiness to make the transition.”

To avoid complications of labor, doctors plan repeat C-sections between 36 and 40 weeks, when most fetuses have developed enough to survive outside the womb. Tests are performed to determine the extent of the fetus’s development, but Thorp said those tests are often not sophisticated enough to determine subtle milestones for breathing and processing food independently.

A high proportion of elective Caesarean deliveries in the United States are performed before 39 weeks, the study’s authors wrote in the journal report. “This may be driven by several factors, including a woman’s desire to give birth once term is attained and an obstetrician’s desire to schedule the procedure at a convenient time.”

Such early deliveries, the authors said, should be discouraged, based on the findings of the study.