How soon can you hear the baby's heartbeat with a doppler?

Usually at 12 weeks with proper equipment and technique. If you don't know what you're listening for you could confuse it with your own heartbeat. I had a doppler at 11 weeks and didn't pick up, my u/s showed all was ok. I had my 1st ultrasound at 6 weeks and we could already see the heart beating away. When you go in for your scan, ask the sonographer to show you the heart and if it's beating.