The number of preterm births in the United States increased by more than a third between 1996 and 2004 and Cesarean sections accounted for the vast majority of the increase, researchers said Wednesday.

Most of the increase involved what physicians call late-preterm babies, those born after 34 to 36 weeks of gestation rather than the normal full term of 38 to 42 weeks.

Physicians are concerned about the growing number of late-preterm babies — which now account for 72 percent of all preemies — because recent studies have shown serious health risks for them, including immature organs, breathing problems, feeding problems, difficulties regulating body temperature, jaundice and a three-fold increase in death during the first year of life.

The number of deaths is small, about three per 1,000 late-preterm births. "But it is a serious problem," said Dr. Alan R. Fleischman, medical director of the March of Dimes.

Many of the other problems eventually resolve themselves, but cost of the added care can be substantial.

According to the Institute of Medicine, medical costs associated with premature delivery total $26 billion per year in this country. More than 50 percent of those costs are for complications involving late-preterm babies, Fleischman said.

While many of the C-sections are medically necessary, experts fear that growing numbers are the result of physicians' fears of lawsuits arising from complications during labor and the mothers' desire to schedule the births at a convenient time.

Since 1975, the proportion of infants delivered by C-section have increased from 5 percent to 30 percent, making the procedure now the most common surgical procedure for women.

"To do an elective C-section without a medical indication 20 years ago would have been unheard of," said Dr. Bruce Flamm, an obstetrician/gynecologist at the University of California, Irvine, who was not involved in the study. "Ten years ago, it was very controversial. Now it is much less controversial. Many are done today because the mother wants it."

The American College of Obstetrics and Gynecologists has guidelines firmly stating that C-sections should not be performed unless there are medical indications for it.

The new study, published in the June issue of the journal Clinics in Perinatology, was conducted by researchers from the March of Dimes and the federal Centers for Disease Control and Prevention. Preterm births now account for more than 520,000 babies each year — or one in every eight children born in the U.S.

Prematurity, especially earlier than 34 weeks, is associated with a broad spectrum of problems, including increased mortality, delays in development, neurological problems, lower IQs and a host of other complications.

In the new study, researchers focused on singleton births because multiple births are known to frequently involve medical complications requiring C-sections.

They found that during the nine years of the study period, the number of singleton preterm births increased by nearly 60,000 per year, and 92 percent of the extra infants were delivered by C-section.

The medical records used in the study typically did not give a reason for the C-section and the researchers could only speculate about causes.

One partial explanation is revealed by the fact that C-section rates are increasing most rapidly among blacks and and Hispanics, said Dr. Khalil Tabsh, chief of the division of obstetrics at the University of California, Los Angeles' Geffen School of Medicine.

Those groups have higher rates of pregnancy-induced hypertension, diabetes and high-risk pregnancies, he noted. In the past, many women in those groups didn't have access to adequate prenatal care, he said.

"Now that they are getting better care, they are catching up with the whites" in C-section rates, he concluded.