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.

The Spain downgrade helped lift the dollar against the euro and pushed oil prices lower, even as Libyan leader Muammar Gaddafi carried counterattacks deeper into the insurgent heartland. Brent crude futures fell 1.8 percent to $113.85.

Several factors today -- China came out with a trade deficit that certainly surprised the markets and Moody's downgrading Spain's debt -- is also very much in focus. The situation in Libya as Gaddafi seems to be gaining ground against the rebels is also adding to the wall of worries, said Peter Cardillo, chief market economist at Avalon Partners in New York.

This market is on shaky ground as we enter next week's quadruple witching options expiration, which is likely to keep the market on a bumpy path.

S&P 500 futures lost 3.7 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 20 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.

Human Genome Sciences Inc jumped 4.4 percent to $26.80 in premarket trade after health officials cleared 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 2.8 percent to $35.50, and Green Mountain surged 23.3 percent to $53.80 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)