Usual for heart rate to decrease?

[Pregnantandfreakingout]

I'm using at at home fetal doppler. I'm not looking for judgement on that but I'm 15w and can easily find the heartbeat now.

My question is the other day the heart rate was 140 -145 but now its 135-140, is that normal that it has decreased or a sign of a problem and possible miscarriage?

Just to add I've taken the reading several times and it's consistently 135 to 140 today, eg After eating, before eating etc

Both figures are in the healthy range, and it can vary throughout pregnancy. Maybe the higher figure caught baby moving about a bit. Maybe the doppler just isn't that precise. Baby has a heartbeat, which is in the healthy range, that's the key thing. Try to step away from the doppler if you can. Your midwife may check the heartbeat at 16 weeks (depends on policy, it can be very hard to accurately measure, even for a trained mw with excellent equipment). If worried, perhaps you could book a private scan? Or text/call mw for reassurance tomorrow?