I agree with all the other ladies have said. The fetal heartbeat is typically detected between 5 1/2 and 6 1/2 weeks. Now, it is common to have dates be off and, depending on the quality of the equipment being used, for a heartbeat not to be detected until later on. I think you are doing the wise thing..a repeat ultrasound. I am so sorry you are going through this and I wish you well.
I went 5 weeks without knowing my baby was dead. I had no spotting no cramping..nothing. Then I started having light pink spotting and went to er and then I found out baby had died 5 weeks earlier. That was the most confusing and hardest moment of my life. I could not understand how or why. Especially concerning God, how could he let me think I was 12 weeks and the whole time my baby was gone?? Ugh I'm still struggling.
It is possible that your dates are off which happens often in a natural pregnancy. That is a definite possibility.
On the other end it may be a missed miscarriage which means that the development stopped but your body has yet to recognize it hence no spotting or cramping. This too can be called a blighted ovum- where a fetus fails to develop.
You can wait a weeks or two and then go for another ultrasound which should give you a definitive answer. Also, you may end up miscarrying during that 2 weeks.
If you are indeed over 7 weeks there should be a heartbeat.
Good luck to you.