Confused about my SILs scan results. She was around 6 weeks and 5 days pregnant when she had her scan.

Baby measured at 6 weeks 2 days, but the sac measured at 8 weeks! Why such a discrepancy in dates?

And we are all a little hesitant about getting too excited as the babys heart rate was 87bpm at the scan. I've heard that anything under 90bpm at that gestation generally means high chance of MC?

Can anyone please give me any hope and positive stories that I can pass on to my sis in law please? She's stressing and that can not be good for her or the baby.