* Economic data points to slowdown in U.S. growth
* Shares of eBay slide after forecast disappoints
* UnitedHealth profit falls, shares drop
* Dow off 0.6 pct, S&P 500 off 0.7 pct, Nasdaq off 1.2 pct
By Caroline Valetkevitch
NEW YORK, April 18 (Reuters) - U.S. stocks fell on Thursday and the S&P 500 closed below a key technical level after disappointing forecasts from eBay and other companies, casting doubt on the market's recent strength.
The S&P 500 ended below its 50-day moving average of 1,543.04 for the first time this year, giving more weight to views that the market's recent rally is losing momentum, particularly after two days of sharp declines earlier this week.
The Nasdaq 100 and the Russell 2000 indexes both have closed below their 50-day averages this week, adding to the overall technical pressure on the market.
Technology led the day's fall. Shares of eBay dropped 5.9 percent to $52.82, a day after the e-commerce company posted results and gave a disappointing earnings forecast for the second quarter. The S&P technology sector index shed 1.4 percent.
Apple shares extended their slide from Wednesday, when the stock broke below $400 on an intraday basis for the first time since December 2011. The stock tumbled 2.7 percent to $392.05 on Thursday.
The CBOE Volatility Index, Wall Street's fear index, gained 6.4 percent to 17.56. The VIX is up roughly 46 percent for the week so far. It still remains well below its recent highs, but the gains could signal a change in the market trend.
"There's definitely technical damage," said Bruce Zaro, chief technical strategist at Delta Global Asset Management in Boston. "I think that the period we had that had volatility tamped way down has likely ended."
Stocks have rallied for much of the year on views that the U.S. economy is strengthening and the Federal Reserve will keep its economic stimulus in place.
More recent data on the economy has been less upbeat. On Wednesday, reports showed factory activity in the Mid-Atlantic region cooled in April and the index of leading indicators, a gauge of future U.S. economic activity, fell in March for the first time in seven months.
The Dow Jones industrial average slid 81.45 points, or 0.56 percent, to end at 14,537.14. The Standard & Poor's 500 Index dropped 10.40 points, or 0.67 percent, to 1,541.61. The Nasdaq Composite Index fell 38.31 points, or 1.20 percent, to close at 3,166.36.
After the bell, a number of high-profile tech companies reported results, including International Business Machines , whose shares fell 3.5 percent to $200.01 after its earnings missed analysts' expectations.
Google shares rose 0.7 percent to $770.97 after the bell following the release of its results, while shares of Microsoft shot up 2.2 percent to $29.43 after its results.
During Thursday's regular session, volume was roughly 7.05 billion shares traded on the New York Stock Exchange, the Nasdaq and the NYSE MKT. In comparison, the average daily closing volume is about 6.36 billion this year.
Decliners beat advancers by about 16 to 13 on the NYSE and by about 5 to 3 on Nasdaq.
Volume has been heavier on negative days this week, as many investors have anticipated a pullback for some time after stocks' strong run to start the year, and moved quickly to book profits.
The S&P 500's moving average was also the floor of the trading range during the last month, making 1,543 a key technical support, according to Richard Ross, global technical strategist at Auerbach Grayson in New York.
Ross and others noted that the S&P 500 has posted negative second quarters in the last three years.
The S&P 500's healthcare sector also experienced big declines, with UnitedHealth Group Inc down 3.8 percent at $59.69, after the insurer lowered its 2013 revenue outlook.
Other decliners included Morgan Stanley, whose shares dropped 5.4 percent to $20.31 after the bank reported revenue from fixed-income and commodities trading fell sharply from a year earlier. Shares of Bank of America, which posted disappointing results on Wednesday, fell 2.2 percent to $11.44 on Thursday.
S&P 500 earnings are expected to have risen 1.9 percent in the first quarter, up from the 1.5 percent estimate at the start of the month, based on actual results from 82 companies and estimates for the rest, according to Thomson Reuters data as of Thursday morning.
Of companies that have reported, 72 percent have topped analysts' expectations, but only 43.9 percent have beaten revenue forecasts.