Wow. Yeah, switch to another clinic. That is a bunch of ridiculous info you were given. I went in this past Monday for my heart beat scan. For me it was 6+4 and baby was measuring 3mm (a few days behind) and had a heart beat of 116 BPM. I was really worried, but my doctor said that at this point, a few days behind doesn't mean anything. As long as it's less than a week behind, there is no reason to worry. He also said 116 BPM is ideal for my date, and that's the main indicator. I did IVF, so that appt was with the high risk perinatologist. He was so happy with how things went, he moved me back down to low risk and said he doesn't need to see me again unless something new comes up, so now I'm back to normal care with the OB. (I saw her yesterday, and she too said that a few days off is completely normal, even though I know my exact conception date.)
Of course, I'm still worried anyway! I go in for my formal pregnancy intake next Thursday at 8 weeks. I will be very happy to see my baby on ultrasound and see that he/she is growing normally.