WASHINGTON — Medicaid expansion has given a budget boost to participating states, mostly by allowing them to use federal money instead of state dollars to care for pregnant women, inmates, and people with mental illness, disabilities, HIV/AIDS, and breast and cervical cancer, according to two new reports.

States that levy assessments and fees on health care providers, which have collected higher revenues as a result of expansion, have reaped extra benefits.

Under the Affordable Care Act, states can choose to expand Medicaid, the joint state-federal health program for the poor, to adults with incomes up to 138 percent of the federal poverty level — $16,242 for an individual. The federal government is paying the entire bill for the new Medicaid beneficiaries through 2016, a declining share over the following three years, and 90 percent thereafter.

Twenty-nine states and the District of Columbia have opted to expand. One argument against expanding Medicaid is that doing so will put a strain on state budgets, either now or in the future. But the two reports, one commissioned by the Robert Wood Johnson Foundation (RWJ) and the other by the Kaiser Family Foundation (KFF), suggest the opposite.

The RWJ report examined eight Medicaid expansion states: Arkansas, Colorado, Kentucky, Michigan, New Mexico, Oregon, Washington and West Virginia. It found those states, selected for their regional and demographic diversity, will save a total of $1.8 billion through Medicaid expansion by the end of 2015.

“I don’t know any other way of describing it except as a win-win for us,” said Audrey Tayse Haynes, secretary of the Kentucky Cabinet for Health and Family Services, which expects to have saved nearly $109 million by July 1.

The KFF analysis, which looked at Connecticut, New Mexico and Washington state, concluded those states have experienced “state savings and revenue gains with limited costs resulting from expansion, even as some potential fiscal gains have not yet been tracked.”

Historically, many states have used their own money to pay for programs and services for people without insurance. In expansion states, many of those uninsured now are getting coverage through Medicaid, which means the federal government is paying for their care. The states that were the most generous to the uninsured before expansion are garnering the biggest benefits now.

“You can say states that had more significant programs for the uninsured before expansion are indeed going to see the most significant savings” in certain budgetary areas, said Patricia Boozang, one of the co-authors of the RWJ study.