Mississippi is one of 12 states that continue to block a broad expansion of Medicaid under the Affordable Care Act, more than a decade after the law was passed. That has left tens of thousands of women of reproductive age — 43,000 in Mississippi alone — without access to routine medical care before they get pregnant that helps ensure healthy outcomes when they do. Many face barriers in accessing family planning services to prevent unwanted pregnancies in the first place.

“We’re talking about 800,000 women of childbearing age, two-thirds of them women of color, who are falling between the cracks right now,” said Maya Wiley, president of the Leadership Conference on Civil and Human Rights in Washington. “And so many of these states are the same states trying to force them to have babies against their will.”

Republicans who oppose a broad expansion of Medicaid often cite cost concerns, although the law requires the federal government to cover 90 percent of the expense. They include Mr. Reeves and Philip Gunn, the House speaker, who blocked the extension of Medicaid for new mothers. Through their offices, both declined repeated requests to comment.