I had this same exact problem at 7 weeks (with a Clomid baby) and everything turned out just fine-- in fact, my "problem pregnancy" is now screeching in my ear for me to get of the computer! :-) I won't tell you not to worry because nothing I say will keep you from worrying, but the heartbeat is a great sign and you have to remember that all the ultrasound work is as much art as science. They will probably have you back in in a week or so and remeasure and my guess is that everything will be fine by then. Make sure you are taking the best care of yourself as you can (including trying to keep your stress level as low as possible) and keep yourself mentally busy while you wait for the second ultrasound. Good luck, and let us know what happens!