Why would Caesarean birth rates vary from as little as 7 percent at some U.S. hospitals to 70 percent at others?

That’s the question raised by a University of Minnesota study released Monday, which found that C-section rates vary tenfold across the country.

The study is one of the largest of its kind to suggest that some C-sections are being performed for questionable reasons — inflating health costs and putting women and their babies at unnecessary risk.

“We were quite surprised,” said Katy Kozhimannil, an assistant professor of public health who led the study, which was published in the journal Health Affairs. “We expected pretty wide variation,” she said, but not tenfold.

The new study, which examined nearly 600 U.S. hospitals with at least 100 births in 2009, found that the variations were “striking in their magnitude,” even among medical centers of the same size and similar patient populations.

Among low-risk women, researchers found a 15-fold variation in C-section rates, according to Kozhimannil. In that group — women with normal, full-term pregnancies and no prior C-section — the rates ranged from 2.4 percent of births to 36.6 percent.

Kozhimannil said they couldn’t tell from the data what reasons doctors gave for ordering the C-sections. But she called the sheer size of the gap alarming.

“It should be concerning to all women to see this kind of variability,” she said.

Dr. Stan Davis, an obstetrician and consultant to the Minnesota Hospital Association, said he, too, was alarmed by the findings.

“What it tells me is that people really aren’t following evidence-based guidelines for when they should be doing Caesareans,” he said. “When you see a tenfold difference in some kind of statistic, it really tells you there’s something drastically wrong.”

More than 1.6 million women give birth by Caesarean section each year, making it the most common operation at U.S. hospitals, according to the report. The rates have been climbing steadily, from 21 percent of all births in 1996 to 33 percent in 2011.

In the right situations, C-sections “save lives, and everyone who needs one should absolutely have one,” Kozhimannil said.

But the variations — among hospitals as well as parts of the country — raise questions about which, and how many, are truly medically necessary.

“That is the issue with variation,” said Kozhimannil, who specializes in women’s health issues. “You have potential for overuse and for underuse, both of which have potential health and cost implications.”

Caesarean sections cost thousands of dollars more than normal deliveries, and expose both mother and child to higher risk of complications, such as infections and respiratory problems.

Already, many hospitals are taking steps to deter doctors from scheduling C-sections and inductions for nonmedical reasons, said Tania Daniels, vice president of patient safety at the Minnesota Hospital Association. But she said surgical deliveries remain frequent among women with prior C-sections who are thus at higher risk for complications.

Experts have cited a rise in risk factors such as obesity and diabetes, as well as threat of malpractice suits, for the steady increase in C-sections. But Kozhimannil said her research suggests they don’t explain the wide variations from hospital to hospital.

There’s no easy solution, she said. But in the report, she and her colleagues call for hospitals to publicly disclose their C-section rates, to allow patients to compare and to spark discussion between patients and doctors.

Last month, MN Community Measurement disclosed the C-section rates at hundreds of Minnesota clinics in its 2012 health quality report, and also found wide variations.

But there’s no similar public report about Minnesota hospitals.