U.S. fixed mortgage rates fell slightly as the economic growth was much slower than market expectations in the first quarter of this year, said the Primary Mortgage Market Survey released Thursday by Freddie Mac. The U.S. mortgage giant said the 30-year fixed-rate mortgage ( FRM) declined to 4.29 percent in the week ending Thursday from 4. 33 percent in the previous week. The 15-year FRM, a popular guide for those looking to refinance, edged down to 3.38 percent this week from 3.39 percent in the prior week. "Mortgage rates were down slightly following the release of real GDP estimates for the first quarter of the year which rose 0. 1 percent and fell well short of market expectations," said Freddie Mac's chief economist Frank Nothaft in a statement. "Meanwhile, the pending home sales index rose in March, ending eight consecutive months of decline and the S&P/Case-Shiller 20- city composite house price index rose 12.9 percent over the 12 months ending in February 2014," said Nothaft. The five-year Treasury-indexed hybrid adjustable-rate mortgage (ARM) edged up to 3.05 percent this week from last week's 3.03 percent, while the one-year Treasury-indexed ARM rose to 2.45 percent this week from 2.44 percent in the previous week.