This is very common. I went in at 6 weeks and we saw no baby until I was 7.5 weeks. I wouldn't worry. They should schedule you another ultrasound.
In my first pregnancy I went at 5 weeks for an ultrasound also, and was told the same thing. Only saw a sac but couldn't see a baby. This was just too early, and I was sent back a couple of weeks later for another Ultrasound. Try not to worry. I had a baby after all despite seeing nothing but a sac at 5 weeks! This current pregnancy my OB told me not to even set foot into an ultrasound clinic until I was 6 weeks plus. I wouldn't worry too much. It was totally normal for me.
I hope that everything turns out just fine. Good luck!
Sincerely,
Sandi (Dragon1973)
MedHelp Down syndrome Community Leader
& Ds Group Forum Founder/Moderator
This same thing happened to my friend and she is now 13 weeks pregnant. Sometimes you might think you are further along than you actually are. Did they schedule you for another u/s. Usually the baby won't show up on u/s until 5 weeks.