Possible miscarriage

I have been seen almost weekly by RE since learning I was pregnant again after 3 miscarriages. I should be about 7 weeks 5 days. I had an ultrasound 2 weeks ago and everything was normal. Today I had another ultrasound and was told the heart beat was irregular and at one point dropped to 85 bpm. I was also told the baby is measuring a week behind. I asked if this could be due to longer cycles or ovulating later. She said no, because in the 2 weeks since my last ultrasound I only made about a weeks progress. I go back in a week for another ultrasound but was told to prepare for the worst. Just checking to see if anyone else has experienced anything similar and what the outcome was??