NEW YORK — U.S. stocks are opening lower and are headed for their biggest weekly loss since April.The Dow Jones industrial average fell 40 points, or 0.2 percent, to 16,873 in the first few minutes of trading Friday.The Standard & Poor's 500 index lost four points, or 0.2 percent, to 1,960. The Nasdaq composite fell four points, or 0.1 percent, to 4,391.Investors were assessing company earnings. Fastenal, which makes industrial fasteners, fell 5 percent after reporting revenue that fell short of analysts' estimates.The S&P 500 index is poised for its biggest weekly loss since April after closing out the previous week at a record high. It's down 1.2 percent so far this week.Bond prices rose. The yield on the 10-year Treasury note fell to 2.52 percent.