Just wondering when you can expect to hear bub's heartbeat with a doppler?

I had an appointment with my GP today and he couldn't find bub's heartbeat. He wasn't worried at all and said that because I'm so tall (6 foot) that bub is possibly too low down still for the doppler to work.

I'm trying so hard not to worry but my ms has suddenly lessened and I'm scared it means my levels are dropping.

Any reassurance would be greatly appreciated!