Posts like these make me glad I waited til 13 weeks to get my first scan. I was already waiting and freaking out, and the lack of embryo visual would have stressed me out. Be calm, and try not to stress out. Good luck.
Be positive girly. 6 weeks is quite early to see anything. My doctor told me not to even bother until 8 weeks. Seeing the sack and yoke is all in order is good. Baby's environment is just what it needs to be. These doctors know what they are doing, best leave the worrying to them. Try reading a book or getting in to a good TV series to get your mind off it.
You may have your dates wrong even if your periods are regular happened to me, worrying won't solve anything only make things worse so think positive until you get your scan next week good luck ;)
You need to relax for the baby's health. It's true at six weeks sometimes you can see a baby and sometimes not. For that reason my doctors office won't do a dating scan before eight weeks after your missed period.