no yolk sac 6w4d

Mi
I had my first ultrasound yesterday, measuring 6w4d and saw the heart beating at 122bpm. My doctor called me in today to tell me there was a visible fetal pole but they couldn't see a yolk sac which is abnormal. I had a blood test today at 6w5d and my HCG levels were at 90,000.... but I have hardly any symptoms! 
I have another ultrasound this week but does anyone have any positive stories about these things or should I be preparing for a miscarriage? 
Thanks