Stocks seesawed between modest gains and losses on Wednesday ahead of the Federal Reserve's assessment of the economy and any plans for dealing with renewed weakness.

Supporting the market, shares of economic bellwether FedEx Corp rose 2.2 percent to $91.11 after the shipping group reported a strong fourth-quarter profit and forecast robust 2012 earnings.

The Fed is likely to acknowledge weakness in the U.S. economy and reiterate its commitment to keeping interest rates low for an extended period, according to analysts. Investors will look for clues on new measures to support the economy as the Fed's second quantitative easing program ends this month.

There may be a question about the possibility of QE3 and the way he answers will be key, said John Canally, investment strategist and economist for LPL Financial Boston. If Bernanke's tone on the subject changes, then the market will react instantly.

The Dow Jones industrial average <.DJI> was down 28.49 points, or 0.23 percent, at 12,161.52. The Standard & Poor's 500 Index <.SPX> dipped 1.70 points, or 0.13 percent, at 1,293.82. The Nasdaq Composite Index <.IXIC> took off 6.10 points, or 0.23 percent, at 2,681.16.

U.S. stocks posted gains for the fourth straight day on Tuesday on expectations the Greek prime minister would survive a confidence vote, a key hurdle to avoiding a debt default, adding momentum to a recent rebound.

The Nasdaq posted its biggest percentage gain since October on Tuesday, while the S&P 500 marked its best day in two months in what traders see as continued short-term buying from deeply oversold levels.

Adobe Systems Inc reported a 54 percent jump in quarterly profit late Tuesday, but warned of weakness in European demand. Its shares fell 4.7 percent to $30.50.

Net short positions by hedge funds on the S&P 500 have recently increased, according to Societe Generale cross-asset research.

A statement from the Federal Open Market Committee is due at 12:30 p.m. EDT, followed by Bernanke's press conference at 2:15 p.m. EDT.

(Reporting by Angela Moon; editing by Jeffrey Benkoe)