Anybody know if this is a bad sign?

I went in for an ultrasound 10 days ago & baby measured 5W5D with no heartbeat. I came in today 10 days later for another ultrasound and we saw a heartbeat. It was low though, 102 bpm. I also measured at 6W today which is not significant growth. (2 days of growth in a ten day period.) Should I be worried for the worst? Doctor says it can go either way & sometimes fetuses can catch up on growth but he is concerned & wants to measure again in four days. I’m looking for similar experiences & what your outcomes were. My bigger concern is the lack of growth, not the heart rate.