Understanding a Missing Fetal Heartbeat: Causes & Next Steps

Question
I am 36yo and this is my first pregnancy.  My LMP started on 7/21/05 but my cycles are very irregular, ranging from 31-65 days due to PCOS.  My DR and I have pinpointed a conception date of about 8/21/05 and she feels that I am 10 weeks along with a fetal age around 8 weeks.  I went for a prenatal exam on 10/18/05 and had about a 2 hour period of spotting (light brown) right before my appt which the DR said is fairly normal.  She tried a doppler that day but did not find a heartbeat which she said is also normal with a fetal age of 8 weeks, using 10 weeks as my LMP date.  She ordered an ultrasound for more accurate dating and I had that done yesterday.  First, an abdominal which showed a fetus. Then a transvaginal US showed a fetus of about 7.5 weeks fetal age but no heartbeat and a yolk sac thats a little large.  She told me that she is 80% sure that I have started to miscarry and is doing a repeat ultrasound next wednesday to confirm.  What are the chances of my pregnancy actually being fine and that not seeing a fetal heartbeat at this stage is somewhat normal?  Is it possible that the babys arm or leg was in the way and the heartbeat could not be seen.  I haven't had any more spotting in addition to the short spotting in 10/18/05.
Thank you!

Answer
After 6 weeks (from the first day of your last period) we usually see a fetus and a fetal heartbeat on sonographic exam. If the ultrasound showed a 7.5 week fetus but no fetal heartbeat, it is not promising for a viable pregnancy. If, on the next sonogram, you still do not have a heartbeat, I'm afraid it is, indeed, a miscarriage.