Washington County recorded an unusually high number of preterm births in 2013, according to a new March of Dimes report that is surprising Minnesota officials because the suburban county is largely white and upper-income, while the public health problem tends to be more common in poor and minority communities.

Neighboring Ramsey County and the city of St. Paul, which are more diverse racially and economically, posted a lower rate — just 8 percent, compared with 9.5 percent in Washington County.

St. Paul’s rate was third-lowest among the nation’s 100 largest cities, according to the report, released this morning.

“We really need to dig deeper into the data,” said Michelle Gogerty, interim program director for the March of Dimes Minnesota chapter. “I don’t have a direct correlation of what’s happening in one county vs. another. We just need to use this as a starting point.”

The good news is that Minnesota’s overall preterm birthrate of 8.7 percent is below the national average of 9.6 percent, earning the state a B grade from the March of Dimes. Even Washington County’s rate was below the U.S. average.

Preterm refers to any birth occurring before the 37th week of pregnancy, and is associated with higher rates of birth defects and learning disabilities in children. The preterm birthrate also is a proxy for the overall health of a community, because it typically reflects public health challenges such as diabetes, hypertension, smoking and drug abuse.

Minnesota hospitals have been among the most aggressive in the nation at reducing preterm births by eliminating early surgical or induced deliveries that are deemed elective. Most Minnesota providers even discourage elective deliveries until 39 weeks gestation, and that trend is largely responsible for the success in the state and Ramsey County, said Kathy Schoenbeck, manager of the Mother Baby Center, created by United and Children’s hospitals in St. Paul.

“If the mom is less than 39 weeks, she can’t just say, ‘I want to schedule an induction to have my baby now, because I’m tired of being pregnant,’ ” she said.

Such precautions extend to mothers living in Washington County, which makes the high rate in that county puzzling. Adding to the mystery is the fact that Washington County is 87 percent white, but the March of Dimes report showed the highest preterm birthrates in the state among American Indian mothers (11.6 percent) and black mothers (10.2 percent). Ramsey County, by contrast, is 70 percent white.

Washington County’s own data has shown much lower preterm birthrates, suggesting there might be errors in the March of Dimes numbers, said Lowell Johnson, director of the county’s Department of Public Health and Environment. He said, for example, the data might not have adjusted for multiple births, which are more common among upper-income whites and also result in preterm deliveries.

Nevertheless, “this has got our attention,” Johnson said, “and we’re going to look at this and try to determine if it is a methodology issue in the calculations or if we have a different trend in actual birth outcomes. And if so, we’re going to go to work on that.”