6w5d heart rate 80-100 should I be worried?

I had my first u/s today and heard little Roo's heartbeat for the first time. Dr said baby measured and looked great  . I asked what the heart rate was and he said quickly oh between 80-100 I said that sounded low and he said it's fine not to worry. I'm wondering if he actually measured the BPM or just gave me what he thought was the healthy range. I see people here posting much higher numbers. I'm so worried about having an MC but Dr was not concerned at all.