Ultrasound question

Hi girls,

Does anyone know how the baby’s heart rate is detected through an ultrasound? I ended up in emerge and since I have a rapid heart rate when the nurse took my pulse it was 142. I then had my ultrasound done and the baby’s heart rate was 142. They didn’t allow me to see the screen so I have no idea what it looked like but I’m just scared that they mistook my heart rate for baby’s.