The U.S. maternal mortality rate has continued to soar over the past decade, even though it was been plummeting globally for 15 years, a new study finds.

Global death rates fell by more than a third from 2000 to 2015, but the United States is one of the few countries in the world that have gone against the grain, The New York Times reports.

According to the Institute of Health Metrics and Evaluation — a research group funded by the Gates Foundation and based at the University of Washington — American maternal mortality have risen from 23 deaths per 100,000 live births to 28 since 2005.

In addition, the American death rate is up by more than half since 1990, according to the institute, which uses many data sources, including countries’ vital records systems, to calculate hundreds of health measures.

Researchers say the increase in recent years has been driven by heart problems and other chronic medical conditions, like diabetes, which has increased sharply.

They added that an increase in obesity — particularly among poor black women, who have much higher rates of maternal mortality than whites — may be contributing to the problem.