M.
Don't worry - with my first baby I had an ultra sound at 5wks, 7wks and 10 wks (because of previous complications). They only saw the sac in week 5 and 7 and had to wait 'til wk 10 to know if the baby was "viable". My little one is now 4 years old, and they couldn't detect her right away. If there was a problem, the ultra sound technician would have brought a doctor in immediately to talk to you.
Sounds like your preggo hormones are getting to you and making you worry unnecessarily. Between that and your nausea, sounds like your hormones are multiplying. :)
I hope you have a happy, healthy pregnancy and try to relax! (easier said than done)