One in nine babies in Oklahoma was born too early, or before 37 weeks gestation, in 2018, prompting March of Dimes to give the state a D- on its annual report card.

The report, released Monday, found a preterm birth rate of 11.4 percent in the state, an increase compared to 11.1 percent in 2017. Overall, the U.S. rate increased for the fourth year in a row, earning it a “C” grade. March of Dimes is a national nonprofit that supports research, education and advocacy related to infant and maternal health.

The report also highlighted racial disparities in the state. In Oklahoma, the preterm birth...