Does anyone know why babies heartbeat is so low????

Im 19 weeks pregnant with my 3rd i suffered a m/c last year and when i got pregnant again i was very anxious as you would be so my midwife suggested i get a doppler which i did, i havent got any problem finding a heartbeat ever but the only thing is tonight when i listened the heartbeat was right down just above my pubic bone is this normal at 19 weeks i thought it would be higher by now??