Miscarriage or No? Confused!

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. My bigger concern is that I measured at 6W today which is not significant growth. 2 days of growth in 10 days...Should I be worried? Doctor says it can go either way but he is concerned & wants to measure again in four days. I’m looking for similar experiences & what your outcomes were.