What are the chances of miscarrying after a fetal heart rate is detected?

I am 7 weeks as of 5/1/09. I have had 3 prior miscarriages and never had a heart beat detected. I have been in for 2 early ultrasounds and the doctors say everything looks great. Having been down this horrid road before, I am hopeful but petrified something bad will happen. I cannot fathom losing another. I was having some cramping, however my doctor said it was due to a cyst that is completely normal and it just means the pregnancy is progressing. I have also had severe morning sickness in which I've had to be put on meds for because I work full time. Any insight would be greatly appreciated. Oh and they said the heart rate was 153 BPM