I don't think your chances are raised because you hear the heartbeat. When I spoke with my doctor about miscarriages. They are caused by problems with the baby or health issues with the mother if your pretty healthy you don't have a lot to worry about. I can understand your fear though because I was the same way at the beginning. You should be fine and If your extremely concerned talk to your doctor about your chances.
One in five during tje first trimester of 20% the risk goes does significantly after the first trimester but there is still a risk