WASHINGTON — U.S. economic growth in the first quarter was revised down to a lackluster 2 percent — a sharp deceleration and the poorest showing in a year. But economists expect a significant rebound in the current quarter, forecasting a sizzling growth rate of 4 percent or more.

The Commerce Department said Thursday in its final estimate for the January-March quarter that gross domestic product, the country’s total output of goods and services, was even weaker than previously thought. Lower numbers from consumer spending and business inventories helped trim 0.2 of a percentage point off last month’s estimate that GDP had grown 2.2 percent in the January-March period.