Hartford, Conn.-based Aetna will take on $2.5 billion of Coventry’s debt. (Bob Child/ASSOCIATED PRESS)

Aetna announced Monday plans to acquire Bethesda-based Coventry Health Care in a deal valued at $5.7 billion, part of the managed-care giant’s effort to beef up its Medicare and Medicaid programs.

The purchase comes as insurance companies race to position themselves for a broad expansion of health-care coverage slated to take effect in 2014, following the Supreme Court’s decision to uphold the comprehensive reforms supported by the Obama administration.

The deal would help Aetna expand into government-based insurance by increasing its presence in smaller markets and adding more than 5 million members to its pool of 36.7 million.

“We think diversification is incredibly important as we head into health-care reform,” Mark Bertolini, chairman, chief executive and president of Aetna, said in a conference call to discuss the deal. “Time was running out for us to consider something like this because we want to have it ready for integration and closed prior to health-care reform 2014 kicking off.”

The Hartford, Conn.-based company will take on $2.5 billion of Coventry’s debt. It plans to pay Coventry stockholders $42.08 per share, a 20 percent premium on Friday’s closing price, with a combination of cash and Aetna stock.

“I wouldn’t call it a no-brainer, but there definitely was an expectation that Aetna needed to pursue an acquisition that would advance its Medicare and Medicaid portfolios,” said David Windley, an analyst at Jefferies & Co. “In buying Coventry, they found a company that very much fits the bill.”

In July, Coventry reported that its Medicaid business had more than doubled in the past year, to include 932,000 members. Membership in prescription drug program Medicare Part D was up 30 percent, while Medicare Advantage membership climbed 16 percent.

“Coventry’s historic strength with small groups and individuals will balance Aetna’s strength with larger groups,” Bertolini said.

The deal comes on the heels of similar high-profile acquisitions. Earlier this year, Cigna finalized its purchase of HealthSpring, a Medicare Advantage plan, for $3.8 billion. In July, WellPoint agreed to acquire Amerigroup, which specializes in Medicaid programs, for $4.46 billion.

“We’re seeing larger managed-care companies reposition themselves based on their needs,” said Thomas Carroll, an analyst at Stifel Nicolaus. “The bigger companies are all in the midst of digesting acquisitions now. I don’t think we will necessarily see more deals like this until at least 2014.”

Aetna’s purchase of Coventry has been approved by the boards of both companies, and is slated to close in mid-2013.