WASHINGTON (MarketWatch) - The U.S. economy grew at an annual 3.4% pace in the third quarter instead of 3.5%, revised government data show. Slightly weaker consumer spending mostly accounted for the downward revision, the Commerce Department said. Consumer spending, the main engine of U.S. growth, rose 3.5% vs. an earlier 3.6% estimate. This was offset partly by higher inventory growth as businesses boosted stockpiles ahead of threatened trade tariffs. The data confirm there were large but offsetting swings in inventories, which added 2.3 percentage points to growth, and the trade balance, which subtracted 2 percentage points. Looking ahead, economists surveyed by MarketWatch predict gross domestic product will slow to a 2.6% annual rate in the fourth quarter, the slowest pace since the first three months of the year.