What Causes Lack Of Fetal Heart Beat On Pregnancy Scan?

I will be 7 weeks pregnant this Thursday. I had a vaginal ultrasound on Monday and the doctor did not see a heartbeat. He told me to expect the worst but that he would do another ultrasound on Friday. I have had mild spotting (mostly brown for about a week). My hormone levels have been good so far, though he did want to see my hcg levels rise even higher (they were 4700 at about 6 weeks and my progesterone was 23). He told me to do another bloodtest to see if the hormones are continuing to rise. This morning I woke up to heavy bleeding and passed some pretty large clots. I was convinced that I miscarried. The doctor called me today with results of the bloodtest I did yesterday. He said that the hormone levels look good, but that does not necessarily mean that the embryo is developing normally. Since I am RH negative, I am going in tomorrow for a Rhogham shot and the doctor may do another ultrasound to see if I have miscarried. If the embryo is still there, is there a chance it is still too early to see the heartbeat? I should add that last week he did see the gestational sac in the uterus, as well as afetal pole.