Jan. 2 (Bloomberg) — Emergency rooms may be a preferred choice for care among the 3.9 million people newly enrolled in the U.S. Medicaid program for the poor, a finding from a study that suggests Obamacare's costs may be higher-than-expected.

The concern is being raised by economists who said a state Medicaid expansion in Oregon five years ago led newly insured patients to visit ERs 40 percent more often than the uninsured. That finding, published today in the journal Science, runs counter to government assumptions that the newly insured would choose lower-cost options for care, such as doctors' offices.

More than 19 million people nationwide are projected by the government to join Medicaid this year, a 35 percent jump from last year as the Patient Protection and Affordable Care Act broadens eligibility. The rising participation is projected by the Obama administration to boost spending on the program by 18 percent this year and almost double it to $957 billion by 2021.