Another thing to keep in mind is that even though you and your hubby did the deed 4.5 weeks ago sperm can live in you up to 7 days so your egg may not have been fertilized right away. I also had an ultrasound at 5/6 weeks and only saw the yolk sac. When I went back the next week we saw the fetal pole and the heartbeat and were measuring right where we should based on the first day of my last period. Now I am almost 27 weeks:) Good luck, I am sure all is fine!!
I wouldn't worry...sometimes the measurements are a little off...I just had my 5th baby and many of my pregnancies measured "wrong" but the babies were just fine! I have had 3 ultra-sounds around the 5/6 week mark...it is just too early to be positive about anything...best wishes to you!
Let me tell you a funny story. I have had 3 boys. The pregnancy tests were right and the ultrasounds were right..with only 1 of them !
With my second oldest the home tests said I was pregnant in July of 91.( Therefore I was about 3 weeks ) The hospital tests said negative. I went back a month later and the home tests and lack of period still said prego. The hospital tests...blood work this time...said positive. In September they did an ultrasound...it showed that I was 3 months along ! Then in November they did another one, and by golly....it said that I was 3 months along !!! He was born the end of March 92. ( Mothering sunday lol )
With my oldest one, I did a test in June of 87. Again it showed positive on home test...negative on hospital test. 2 weeks later it showed positive at hospital. He was born in begining of March. They induced me when my doc came back from vacation and noticed I was still prego ! One month overdue there.
The youngest one ? Well you see.. I went to the doctor's office...home test in hand...doctor didn't argue....he was born in middle of March 95.
good luck = )