U.S. stock index futures fell on Thursday after a rating downgrade of Spain rekindled concerns over euro zone debt problems and weak Chinese trade data heightened global growth worries.

Moody's one-notch downgrade of Spain came with a warning that further cuts were possible and on the heels of the agency's downgrade of Greece earlier this week.

China swung in February to an unexpected trade deficit of $7.3 billion, its largest in seven years, but economists said the drop was likely to prove temporary.

S&P 500 futures lost 6.1 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures fell 39 points, and Nasdaq 100 futures dropped 14.25 points.

On the economic front, investors will eye weekly U.S. jobless claims and U.S. international trade data for January at 8:30 a.m. EST <1330 GMT>.

HCA Holdings Inc was expected to begin trading on the New York Stock Exchange after the hospital operator sold more shares than planned in the largest U.S. private-equity backed initial public offering ever.

Health officials cleared Human Genome Sciences Inc's Benlysta treatment for lupus. According to Thomson Reuters consensus forecasts, annual global sales might top $3 billion in 2015.

Starbucks Corp is partnering with Green Mountain Coffee Roasters Inc to take on the fast-growing single-serve market. Starbucks gained 0.6 percent to $34.75, and Green Mountain rose 7.7 percent to $47 in premarket trade.

* European shares hit a five-week low as worries about the health of peripheral euro zone economies grew after the Spain downgrade, while miners were hit hard on the Chinese data. <.EU>

Asian stock markets and copper prices fell.

(Reporting by Chuck Mikolajczak; editing by Jeffrey Benkoe)