I am 19 weeks pregnant and am experiencing a fast heartbeat at random times. Is this normal in a low risk pregnancy? And what is the cause of this?

Fast heart pregnant. Because of the increased blood volume and the shunt of blood across the placenta you may be aware of your heart beat when pregnant. Try count it and note if it is regular or skips. If greater than 120 and or very irregular have your OB send you for a cardiologist evaluation.