Stock index futures added to losses on Friday after data showed the U.S. economy grew less than expected in the second quarter.

Futures had already been lower after lawmakers in Washington delayed a vote on a Republican proposal to raise the government's debt limit.

S&P 500 futures fell 14.8 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 were down 127 points, and Nasdaq 100 futures lost 18 points.

(Reporting by Ryan Vlastelica)