Heart beat 84 good or bad?

I had my third ultrasound @8 weeks today. My last two ultrasounds all they saw was a yolk sac and the doctor basically told me I was going to lose the baby. Well here we are a week later and today there was a heart beat and fetal pole. So excited but the heart beat was only 84 and the doctor is a little concerned about that and I’m measuring at 6 weeks 2 days not 8 weeks. Anyway I’m trying to stay positive anyone have a similar story or situation and go onto have a positive healthy baby? This is will be my rainbow baby after having a miscarriage last May 2018. 🙏🏻🙏🏻❤️🌈