Fetal pole no heart beat

My doctor was really expecting a heartbeat today but there was one. They really don't know how far along I am but my first faint positive was April 15. My hormones have been doubling every 48 hours wondefully. Last I knew they were 14,000. I've had two miscarriages. There was a 2mm fetal pole and yolk sac this time. The last ultrasound they just saw a gestational sac that was two weeks ago. Doctor seemed a little concerned but not overly concerned. I haven't had any bleeding. He said if there isn't a heartbeat next week, the pregnancy isn't viable. Has anyone experienced anything similar? I'm trying to calm my nerves and I'm not having an easy time. Waiting for this is so hard.