After a 13-year climb, C-section rates appear to be leveling off in the U.S., according to a new national study.

Roughly one in three American singletons is born by C-section, according to the study - up 60 percent from the most recent low in 1996. But the rate of C-section deliveries has steadied since 2009, a trend experts say is "good news" for American moms and babies.

"It's good news because there's some inherent risks in C-section deliveries compared to vaginal births," said study author Michelle Osterman, a health statistician at the U.S. Centers for Disease Control and Prevention. "It's a very invasive abdominal surgery, and with that comes risks to both mom baby as well as higher cost."

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But there are ups and downs buried in the apparent plateau. While the rate of C-sections performed at 37 or 38 weeks has fallen 4 percent since 2009, the rate of full-term C-sections performed after 39 weeks has increased 3 percent, the study found.

Osterman said the swings could stem from new guidelines issued by the American College of Obstetricians and Gynecologists, which state that vaginal delivery is "appropriate and should be recommended" in the absence of maternal or fetal complications, and that C-section delivery on maternal request "should not be performed before a gestational age of 39 weeks."

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In another example of canceled out ups and downs, C-section rates decreased by 7 percent among women younger than 25 but rose by at least 3 percent for women 25 and older. The rates also varied by location, with some states, such as New York, Oklahoma and Oregon, seeing decreases in 38-week C-section rates since 2009, and others, such as Maryland, Michigan and California, seeing increases in 39-week C-section rates.

Several states saw both increases and decreases in C-section rates, depending on the timing of the delivery, according to the study.

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The study looked only at singleton births because multiples are more likely to be delivered by C-section out of medical necessity, according to Osterman.