Heart rate

6 week ultrasound today. I thought I'd be more relaxed once I heard a heartbeat but I'm not. Normal at my stage (6 weeks, 4 days) is 120-180. My little one's was 120. Before doing the US my doc said since I'm 6 and a half weeks he would expect it to be around 140. I know it's still healthy and everything else looked good and my doc said everything looks great, he has no concerns...but I just can't feel happy yet. I have another US in 3 weeks. My RE said that at that point if everything looks good I have about a 95% chance of delivering a healthy baby. At this stage of the first trimester it's only 66%. So although I heard lots of good things today, the things that are sticking with me are "low end of normal" and "66%". I don't know how to relax about this.