How accurate are ultrasound with baby’s weight


I’m 38 weeks exact. I left my ultrasound today and they told me my son is weighing 9 pounds!!!! They told me i ether risk pushing him out and soemthing about his arm, or I have a c section! im so damn scared!! I hope and pray they are wrong and he’s at least 7 pounds 8 the most! please tel me how accurate they were with your babies weight based off ultrasound scan !!