Heartbroken & Confused

I went to the Er last night with very light spotting and cramps. Both have gone away now. However, I had an appt. with my OB today & she did an US and said I should prepare for a miscarriage because the baby isn't growing like it should. I should be 7 wks 5 days but after measurement of the baby it said I was 6 wks 1 day. This was my 2nd US. I had one Oct. 28th and was dated at 5 wks 1 day. She says a miscarriage will happen naturally this weekend. But the thing is...there was a heartbeat still!! Am I grasping at straws to hang onto that as a sign that my baby will recover and start growing? Has anyone gone through this and had a successful pregnancy? Please comment. I'm so heartbroken. 💔