Help! Expectant and Heartbroken First Time Mom Here

I had my ultrasound last November 8. Based on the ultra sound results, I am 6 weeks and 6 days pregnant. However my OB told me that it doesn't look good daw because no heartbeat was detected. She wants me to go back a week after. If after a week wala pa rin, she will declare miscarriage daw. Do I have to believe the report of the doctor immediately? Does it happen na 6 weeks na wala pa rin heartbeat yet nagcontinue naman ang pregnancy? Thanks for the inputs.