INDIANAPOLIS – Anthem Inc. said first-quarter earnings jumped 54 percent on strong membership growth and one-time gains, as the rapidly growing health insurer boosted its earnings forecast for the current quarter and full year.

The results, released Wednesday, handily beat Wall Street expectations as Anthem prepares to conclude a merger with WellPoint Health Networks Inc. that will create the nation’s largest managed-care company.

Indianapolis-based Anthem, the licensee for Blue Cross Blue Shield plans in Maine and eight other states, reported a first-quarter net income of $295.6 million for the January-March period, or $2.08 per share. That’s up from $191.7 million, or $1.36 per share, for the same period a year ago. Revenue increased 12 percent to $4.6 billion from $4.1 billion.

This year’s first-quarter profit was boosted 47 cents per share by one-time gains, primarily due to a benefit from a tax law change in Anthem’s home state of Indiana, affecting the state’s high-risk insurance pool. The year-ago period included 18 cents per share in gains.

Excluding such gains, Anthem’s profit last quarter came in at $1.61 per share, beating the consensus estimate of $1.46 of analysts surveyed by Thomson First Call.

Anthem said first-quarter results were driven in part by a 9 percent enrollment gain in its medical plans to 12.5 million members. Income from premiums and Anthem’s medical costs both rose 9.5 percent.

Anthem announced plans in October to buy Thousand Oaks, Calif.-based WellPoint in a cash-and-stock deal that would combine the nation’s two biggest Blue Cross Blue Shield companies.