WASHINGTON — 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 one-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 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," he added, since 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.

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 in the number of twins and multiple births, and evidence that doctors might be opting for a cesarean if women encounter difficulties in the early stages of labor.