Stock index futures fell on Friday as investors turned cautious ahead of a report expected to show slower growth of the U.S. economy in the second quarter.

The Commerce Department's gross domestic product report, the first actual estimate of growth for the April-June period, is due at 8:30 a.m. EDT. The data is expected to show that economic growth slowed in the second quarter as a capital investment drive by business was sated by imports and consumer spending tapered off.

Investors have been rattled by disappointing data recently, and more are talking about the possibility of a double-dip recession.

Other data on tap include the Chicago PMI for July at 9:45 a.m. and a final July reading of the Reuters/University of Michigan consumer sentiment index at 9:55 a.m. Economists in a Reuters survey expect a lower reading of 56.5 for the Chicago business barometer index and a slight rise to 67 for the consumer sentiment index.

S&P 500 futures fell 5.1 points and 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 29 points and Nasdaq 100 futures lost 5.75 points.

Drugmaker Merck & Co reported second-quarter earnings of 86 cents per share, above Wall Street's consensus forecast of 83 cents a share. Insurance brokerage Aon Corp. also posted results ahead of the average analyst estimate.

Oil company Chevron Corp is expected to post results later on Friday.

MetLife Inc , the biggest U.S. life insurance company, reported a second-quarter profit of $1.53 billion late Thursday, helped by higher revenue from sales domestically and abroad.

New York's attorney general has subpoenaed MetLife and Prudential Financial Inc as part of a probe into whether life insurers are defrauding families of deceased military personnel by siphoning off millions of dollars of death benefits for themselves.

Amgen Inc reported better-than-expected second-quarter profit late on Thursday, and the world's largest biotechnology company said the launch of its new osteoporosis drug, Prolia, was progressing as planned.

Li Lu, a Chinese-American investor and hedge fund manager, could be in line to take a top investment role at Warren Buffett's Berkshire Hathaway Inc and even succeed the legendary U.S. investor, the Wall Street Journal reported on Thursday.

(Reporting by Angela Moon; Editing by Padraic Cassidy)