Heartbeat

So I went in for my 20 week scan and baby heartbeat was 158 if I remember but I see many people who’s not as many weeks as I am have very high heartbeats of 180 and so and I feel like mine is so low and should be higher :( I know I shouldn’t be worried if dr didn’t say anything about it but I feel like I’m doing something wrong I’m just paranoid over my first miscarriage