You should not be worried at all. It is normal to only see the sac and yolk sac at five weeks.
5 weeks is definately too early to see the fetus. i had a scan at 5 weeks 2 days and saw a the sacs then went back at 7 weeks and 2 days and there was the fetus and heartbeat. Good Luck
5 weeks is too early to see anything. Ask for a repeat ultrasound in two weeks and you should be able to see the baby and a heartbeat. Good luck.