U.S. stocks traded mostly flat Friday after the economy grew more slowly than previously anticipated at the end of 2014. Economists are now looking ahead to a speech from Fed Chair Janet Yellen before the closing bell Friday, searching for clues as to when the central bank will begin hiking interest rates this year. 

In morning trading Friday, the Dow Jones Industrial Average (INDEXDJX:.DJI), which measures the share prices of 30 large industrial companies, fell 39.93 points, or 0.20 percent, to 17,643.30. The Standard & Poor's 500 (INDEXNASDAQ:.IXIC) lost 2.47 points, or 0.15 percent, to 2,053.70. The Nasdaq composite (INDEXSP:.INX) fell 2.39 points, or 0.05 percent, to 4,861.95.

U.S. gross domestic product, the broadest measure of goods and services produced across the economy, was revised lower to an annualized 2.2 percent in the fourth quarter of last year, down from an initial estimate of 2.6 percent in January, the Commerce Department said in its third and final estimate Friday. Economists had forecast U.S. economic growth to have expanded at a 2.4 percent seasonally adjusted annual rate last quarter compared with a 5 percent surge in the third quarter, according to analysts polled by Thomson Reuters.

Yellen speaks at the Federal Reserve Bank of San Francisco Conference at 3:45 p.m. EDT on Friday, discussing "The New Normal for Monetary Policy." The Fed chair will discuss her own view on how the central bank's policy should evolve as the central prepares to raise interest rates. The Federal Open Market Committee moved one step closer to ending near-zero interest rates in 2015 after the central bank removed "patient" from its monetary policy statement following its March meeting, signaling the Fed is on course to raise interest rates in June at the earliest.

Airstrikes from Saudi Arabia and its Persian Gulf allies this week continues to raise concerns about the security of Middle East oil shipments, as Yemen is on the Bab el-Mandeb Strait, one of the world’s main transit points for seaborne trade. In morning trading Friday, Brent crude, the benchmark for global oil prices, fell more than 1 percent to $50.82 a barrel, for May 15 delivery on the London ICE Futures Exchange. West Texas Intermediate crude, the benchmark for U.S. oil prices, fell more than 1 percent to $58.35 a barrel for May 15 delivery on the New York Mercantile Exchange.