NEW YORK (CNN/Money) - A closely watched measure of consumer confidence in the United States fell to its lowest level in five months in September, according to a published report Friday, despite Wall Street forecasts for a slight gain.

The University of Michigan's consumer sentiment index fell to 88.2 from 89.3 in August, according to market sources quoted by Reuters. Economists, on average, expected a reading of 90, according to a Reuters poll. It was the lowest reading for the index since 86 in April.

The university's "current conditions" index, measuring consumers' feelings about the current state of the economy, fell to 98.9 from 99.7 in August, Reuters said. The "expectations" index, looking into the future, fell to 81.3 from 82.5, according to Reuters.

U.S. stock prices extended their losses after the news, while Treasury bond prices rose.

The report followed the Commerce Department's release of August retail-sales data. Sales rose in the month, but fell short of analysts' expectations.

Wall Street pays close attention to consumer spending, which makes up more than two-thirds of the total economy.

The economy has shown encouraging signs of strength lately, with improvements in factory output and consumer spending, much of it fueled by child tax credit checks mailed in July and August, which coincided with the important back-to-school retail sales period.

The labor market has not yet shown signs of recovery, however, leading some economists to worry the recent bump in economic activity could be short-lived.

Certainly, a lack of job growth has weighed on consumer sentiment, but consumers don't always spend the way they feel -- confidence hit rock-bottom immediately after the Sept. 11, 2001, terror attacks, but consumers still responded eagerly to sales incentives, most notably the zero-percent financing offered by automakers.