5 weeks 6 days last beta didn't quite double was 12000 thento 18000 did ultrasound but didn't see a fetal pole. does this mean a miscarriage?

Not necessarily: Usually we see a fetal pole by the time the HCG reaches 10, 000. However, 5 wks 6days is right at the cusp of being able to visualize an embryo. I would reserve judgement until you are a week or so further along, but be prepared for miscarriage. There is also the remote possibility of twins or a heterotopic pregnancy which could raise the HCG level prior to the ultrasound detecting a fetal pole.