The U.S. economy ran below its average growth trend in March, according to the Federal Reserve Bank of Chicago's national activity index, MarketWatch reported. In March, the index swung to -0.23 from an upwardly revised 0.76 in February, and the three-month moving average fell to -0.01 from 0.12. The index weights 85 different economic indicators and is designed so that readings above zero indicate the economy is growing above average historical trends, and readings below zero indicate the economy is running below-average trend growth. When the three-month moving average moves below -0.70, there's an increasing likelihood a recession has begun, MarketWatch said.