UnitedHealth Group Inc posted slightly lower first-quarter profit on Tuesday as membership in commercial plans for employers fell, but the results topped analysts' expectations, and the insurer backed its 2009 profit forecast.

Net income at the largest U.S. health insurer by market value fell to $984 million, or 81 cents per share, from $994 million, or 78 cents per share, a year earlier, when the company had more shares outstanding.

Analysts on average were expecting 67 cents, according to Reuters Estimates.

Revenue rose 8.4 percent to $22 billion.

UnitedHealth's membership in its commercial plans stood at 25.44 million on March 31, a decline of 905,000 from the end of 2008. Some 445,000 fewer people were enrolled in the more lucrative plans for which the company assumes full insurance risk.

But the decline was less steep than UnitedHealth had feared, as the company retained more customers than expected.

Overall membership stood at 32.43 million, up slightly from a year ago. The company added 200,000 people to its Medicare plans for seniors, already ahead of its 2009 target.

Its medical care ratio, a key measure of premium revenue spent on medical costs, was flat at 82.4 percent. The result was better than the 83.8 percent expected by UBS analysts.

UnitedHealth continues to forecast 2009 earnings of $2.90 to $3.15 per share, saying the broad range reflects the uncertain economic climate.

Shares of Minneapolis-based UnitedHealth have fallen about 9 percent in 2009, compared with a 7 percent drop for the S&P Managed Health Care index <.GSPHMO>, amid broad concerns about the financial impact of U.S. health reform measures.

(Reporting by Lewis Krauskopf; Editing by Lisa Von Ahn)