When can the featal heartbeat be heard with a doppler?

I went for my scan yeasterday and when arranging my next appointment with the midwife was told this is when i would hear the heartbeat. I went to the doctors last Friday and heard it then. When telling the midwife this she laughed at me and said 'if thats what you want to believe then fine but i dont think so.' It really burst my bubble. At what stage in development can it be heard?