Heartbeat with at home doppler?

My baby's heartbeat has been steady between 150 and 160 since finding out I was expecting. I've been able to find her heartbeat since very early on my at home Doppler, and it's always been in the 150's then as well. I just tried finding her heartbeat, and found it, but it was extremely slow. Maybe in the 120's. As I listened and counted, it started speeding up back up to the 160's until I heard her move away and kick. My question is this: is it normal for their heartbeat to drop slightly if they're sleeping, even when still in the womb? I know it's normal for us as adults, but I've never experienced hers dropping. I'm 19 weeks. Thank you!