Fetal heart beat activity can usually be detected by 6 weeks. So you are right on the border where the fetus should have a heart beat. However, the ultrasound
instruments we have are not always sensitive enough to detect the new heart beat. For these reasons, it is worth while just checking in a week to see if your doctor
can detect in then. With that said, miscarriages are very common. Thus it is not out of the realm of possibility that the embryo is no longer viable. Pregnancies end in miscarriage usually because there has been a mistake made in the embryo's genes and the pregnancy self destructs. Most women that have miscarriages go on to have normal pregnancies later. The good news is that if your doctor thought you lost the baby, he or she would probably have ordered some blood tests that would have given more information. For example you could get a quantitative serum HCG test (fancy pregnancy test) which would give your doctor some more information on whether the embryo is still viable. I suggest making sure that you make it to the next appointment for another ultrasound. It is the only way you will know for sure. Good luck.