Stock futures waver as investors weigh GDP

U.S. stock-index futures edged mostly lower Wednesday, with investors cautious ahead of the Federal Reserve's latest meeting and after the first estimate of economic growth for the first three months of the year.

The government estimates the U.S. economy grew 0.1 percent in the first quarter, versus a 1.2 percent estimate.

"I think we knew GDP would be impacted by the weather, it was pretty clearly a terrible number, but you don't hear anybody talking about a double-dip recession or even a recession. The economy was impacted by the weather and will likely improve as the year progresses," said Dan Greenhaus, chief global strategist at BTIG.

In her 2 p.m. ET news conference, Fed Chair Janet Yellen is expected to announce a fourth cut to the central bank's bond-buying program, taking its monthly purchases to $45 billion. She is also likely to discuss the economic conditions under which the Fed might start raising short-term interest rates.

"The FOMC (Federal Open Market Committee) statement tonight after the next $10 billion of quantitative easing tapering is likely to be a little more upbeat in acknowledging that the recent economic weakness was weather-related given the evidence of rebound over the past month or so," said Derek Halpenny of Bank of Tokyo-Mitsubishi in a research note on Wednesday.

Read More Focus is on Fed, but this may move markets instead

A trader works on the floor of the New York Stock Exchange.
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A trader works on the floor of the New York Stock Exchange.

Before the market open, a U.S. private sector report showed the economy created 220,000 jobs in April, beating a 200,000 estimate, according to the ADP.

Companies reporting before Wall Street opens include Thomson Reuters, Time Warner and joint-listed GlaxoSmithKline. The pharmaceutical company missed sales estimates and narrowly beat earnings estimates.

Its first-quarter results were of particular interest given its recent multibillion-dollar asset swaps with Novartis. GSK is selling its cancer business for around $16 billion to its rival, and buying Novartis' $7 billion vaccine unit.

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By CNBC's Katy Barnato