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.
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thomas.maugh@latimes.com
