HCG can vary so much person to person and pregnancy to pregnancy. But they do say that you can't see much by ultrasound until the HCG is over 1000-2000. At 5 weeks it is very common to see just the sac and nothing else. Around 6/7 weeks they should see the fetal pole, and a heartbeat the 8th week. If everything measured correctly for 5 weeks then I think you are in great shape. Happy pregnancy!
When I had a scan at the ER at about 5 weeks all I saw was a gestational sac and yolk sac. By my at 8 week u/s we could see it all, with a very strong hb!
Your HCG is on track and so is what you saw! Stay positive, because all is well!!!!!!
hCG levels in weeks from LMP (gestational age)* :
3 weeks LMP: 5 - 50 mIU/ml
4 weeks LMP: 5 - 426 mIU/ml
5 weeks LMP: 18 - 7,340 mIU/ml
6 weeks LMP: 1,080 - 56,500 mIU/ml
7 - 8 weeks LMP: 7,650 - 229,000 mIU/ml
9 - 12 weeks LMP: 25,700 - 288,000 mIU/ml
13 - 16 weeks LMP: 13,300 - 254,000 mIU/ml
17 - 24 weeks LMP: 4,060 - 165,400 mIU/ml
25 - 40 weeks LMP: 3,640 - 117,000 mIU/ml
Non-pregnant females: <5.0 mIU/ml
Postmenopausal: <9.5 mIU/ml