The number of preterm births in the United States grew by more than a third between 1996 and 2004, and Cesarean sections accounted for the vast majority of the increase, researchers said this week.
Most of the upsurge 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% 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 threefold 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 Foundation.
Many of the other problems eventually resolve themselves, but the cost of the added care can be substantial.
According to the nonprofit Institute of Medicine, based in Washington, medical costs associated with premature delivery total $26 billion per year in this country. More than 50% 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 labor complications and mothers' desire to schedule births at convenient times.
Since 1975, the proportion of infants delivered by C-section has increased from 5% to 30%, making C-sections the most common surgical procedures for women.
"To do an elective C-section without a medical indication 20 years ago would have been unheard of," said Bruce Flamm, an obstetrician-gynecologist at UC 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 nonprofit American College of Obstetricians and Gynecologists has guidelines firmly stating that C-sections should not be performed unless there are medical indications for it.
But Dr. Sarah J. Kilpatrick of the University of Illinois at Chicago, chairwoman of ACOG's committee on obstetric practice, noted that some physicians will deliver a baby early for relatively minor reasons, such as a slight increase in the mother's blood pressure.
The new study, published in the June issue of the quarterly 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 United States.
Prematurity, especially earlier than 34 weeks, is associated with a broad spectrum of problems, including increased mortality, delays in development, neurological problems and lower IQs.
In the new study, researchers focused on single 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 single preterm births increased by nearly 60,000 per year, and 92% of those 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 lies in the fact that C-section rates are increasing most rapidly among African Americans and Latinos, said Dr. Khalil Tabsh, chief of obstetrics at UCLA's 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.