Fetal heart monitors (or Doppler machines) do appear to be safe for the fetus. But, I generally do not recommend them for use at home. In many cases, they cause more stress for expectant mothers than relief. It can be difficult at times to find the fetus' heart rate since the fetus moves around a lot. Also, the fetal heart rate normally has some variation to it, so I've seen mothers get very concerned when the heart rate is higher or lower then they've seen before.