U.S. stock index futures fell on Wednesday after Portugal's sovereign credit rating was downgraded one notch on budget concerns.

The cut by Fitch Ratings to AA- dragged down European shares and took the euro to new 10-month lows against the U.S. dollar. Concerns over high sovereign debt loads among some euro zone nations, including Greece, have plagued equity markets, drawing investors away from risky assets, including stocks.

Fitch warned that further underperformance this year and next could bring another downgrade but added that the government's long-term austerity plan was broadly credible.

I think this is just the beginning of a long process of downgrading a number of major governments until they put their debt in order, said Rick Meckler, president of investment firm LibertyView Capital Management in New York.

It's a negative because it sends the message that investments that were previously thought to be safe could have problems to them.

S&P 500 futures fell 4.2 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 dropped 21 points, and Nasdaq 100 futures shed 6.75 points.

The stronger dollar hit commodity prices, which could weigh on resource companies. The price of oil tumbled 1.7 percent to $80.50 a barrel.

Economic reports on tap include durable goods at 8:30 a.m. EDT and new home sales at 10:00 a.m. EDT, both for February. Durable goods orders are expected to rise 0.7 percent, compared with a gain of 2.6 percent the month before, while new home sales are seen rising to 320,000 from 309,000 in the previous month, according to Reuters data.

Design software maker Adobe Systems Inc gained 5.7 percent to $37.22 in premarket trading, a day after reporting adjusted first-quarter profit that beat expectations and revenue came in above consensus.

MF Global Holdings Ltd shares shot up 15.3 percent to $8.44 after former Goldman Sachs chief executive and former New Jersey governor Jon Corzine was named chairman and CEO of the futures and options broker that has lost money for the last four quarters.

On the downside, Jabil Circuit Inc sank 5.5 percent to $17.34 in late trading Tuesday after the technology distributor reported that quarterly revenue fell short of estimates.

General Mills Inc shares fell 1.4 percent to $72.51 in light premarket trading after the cereal maker reported higher-than-expected quarterly profit.

Stocks rallied Tuesday, led by the technology, industrial and materials sectors, driving the Dow and the S&P 500 to 18-month highs. The Dow chalked up its 10th day of gains out of the past 11 sessions.

(Editing by Jeffrey Benkoe)