Is it common not to see an embryo on a 6 week scan?

I went to the ER on Saturday (I should have been 5 weeks exactly) with cramping and a HR 130. I have never had any bleeding. The ER docs thought my baby was in a tube. So they did a U/S and said that all was good and that the baby was actually 5 weeks and 4 days. When I went to my scheduled U/S on the following Wednesday, the Radiologist thought that the baby was supposed to measure 6 weeks 1 day... she said that there was no heartbeat and that the u/s had not changed that much from Saturday... she called it a failed pregnancy. I went immediately to my fertility doctor, who calculated from my date of ovulation (I had artificial insemination, so I know when i ovulated)... he said that I was 5 weeks and 4 days on WEDNESDAY... but he said you should see a embryo by 5 weeks and 4 days... he said I have a 66% of miscarriage. I have to go back in a week for another u/s... Has anyone else gone through this? Does an u/s really change that much in 4 days?