I may have completed misremembered this but I thought I read that using a doppler machine too often is not good for the baby. I'm open to correction on that
The thing is no one knows. There doesn't seem to be a reason any U/S could cause harm, but you should probably err on the side of caution.
You really have no idea what you are listening to, the sound isn't actually the heart beat, it is the reflection of ultrasound and if it is in the right place then that reflection i of the baby's blood cells flowing through the heart.
Early in pregnancy we talk about a 'heart beat' but the heart starts out as two tubes they go through many changes and is only fully formed by about 10 weeks.
The baby's heart rate is nearly twice that of a normal person. If you picked up a HR of 80 you need immediate advice.
It's nearly tice that of the normal adult HR at rest which means being supine for 10 mins resting, not having had a large meal and not be stressed.
A small, pregnant, woman may well have a HR significantly above the 'average'.
An intelligent person is just as capable as a midwife of using the machine sensibly.
This is nonsense, I'm trained to perform echocardiograms, I'm out of practise but I'm fairly sure I could still make accurate readings but I would not have any idea how to use a stand alone Doppler to get a baby's heart beat.