Bleeding and Clotting is miscarriage is inevitable?

I am 6 weeks pregnant and started bleeding and clotting yesterday so I went to the ER they did a pelvic exam and said my cervix is still closed. They did an ultrasound and said the baby was fine even picked up a heart beat. They also did blood work which was fine. I am still bleeding kind of heavy and passing clots. They said I have a threatened miscarriage. Does this mean a miscarriage is inevitable? DO some woman bleed and clot, and go on to have healthy babies?