Same thing happened to me but I was 10weeks when the baby was fine on u/s. A week later the baby had died. My doctor told me after that u/s that I had about a 1% chance of m/c then. Well I guess I am not very lucky
It took my body three weeks after my baby died to m/c naturally. My water actually broke and I had a mini labor. Not everything came out though and I had to have a D&C still.
I have been PG once since then and it was chemical. I refuse to give up.
From what I have read causes of a m/c after u/s with heartbeat can be due to placental problems, genetic defects, and blood clotting problems with the mother.
I am not sure you ever will get the answer to the question of why. It really stinks