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.

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)