Babys heat rate depends on activity level (just like you and I) so will change constantly. Heart rate will be between 120-180 bbp.
Babies are just like adults. The heart rate will be slower or faster depending on what the baby is doing. If baby is napping or resting its going to be lower vs if baby is bouncing around and kicking it will be higher. Also at the end of pregnancy it is lower than earlier in the pregnancy.
I have only heard my babys heartbeat twice but mine was the same way the first time it was 162 beats then a month later it was 141 beats I asked the dr about it and she said that it is normal for the babies heart beat to fluctuate until about 32 weeks