There was an awful story on the BBC news website today about a mother who was worred about a her baby's movement and didn't seek medical help because she found a heartbeat using her doppler at home. Very sadly, the baby was stillborn. Link here.
I work for a company that makes dopplers and other hospital equipment. I'm not representing them in any way but if you ahve a doppler at home it really should only be used for fun/bonding and not ever to check that your baby is ok. It's really easy for someone without the extensive training that a midwife/doctor has to 'find' a heartbeat when it's actually the mum's heartbeat or the placenta. It's also possible to find the baby's heartbeat but not realise that it's abnormal. If you feel like there's something wrong, always phone your midwife or the hospital. They would so much rather check you over and it be a false alarm than not have you come in and something be the matter.
Sorry to be preachy, it just saddens me so much when I see stories like this.