WASHINGTON (AP) -- More women will be giving birth by C-section for the foreseeable future, government scientists said Monday, releasing a study into the causes of a trend that troubles maternal health experts.

Overall, cesarean deliveries account for about a third of births in the U.S. While much attention has recently focused on women having repeat C-sections, researchers with the National Institutes of Health found that nearly one third of first-time moms delivered by cesarean.

That is "somewhat surprising," said Dr. Jun Zhang, lead author of a study that looked at nearly 230,000 deliveries in 19 hospitals around the country. "It has consequences for future pregnancies."

Many doctors and hospitals follow a policy of "once a cesarean, always a cesarean."

The study also suggested a link between chemically induced labor and higher likelihood of a C-section. Women whose labor was induced were twice as likely to have a cesarean. The authors said more research is needed to clarify if there's a cause-and-effect relationship.

Many medical experts consider cesarean deliveries to be a major component of "overtreatment" in the U.S. -- procedures and tests that provide little or no benefit while subjecting patients to additional risks. Indeed, new clinical recommendations say vaginal birth is safe for most women who've had a first C-section.

But the trend does not appear likely to reverse. Since the mid-1990s, the C-section rate in the U.S. has increased by more than 50 percent.

How low should it be? In Scandinavian countries it hovers in the 20 percent range, with no evidence of ill-effects for mothers or babies, Zhang said. How high can it go? In some countries 60 percent to 70 percent of babies are now delivered surgically.

"I hope that we won't get there," said Zhang. "The upward trajectory seems likely to continue in the near future."

Explaining the increase in C-sections is no simple matter. The study found a variety of reasons, some related, including heavier moms and babies, women giving birth later in life, an increase the number of twins and multiple births, and evidence that doctors may be opting for a cesarean if women encounter difficulties in the early stages of labor.

One factor that made no difference was whether the mother had private health insurance or was covered through a government program like Medicaid.

The study was published in the American Journal of Obstetrics & Gynecology.

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