feeling concerned/cautiously optimistic

I just had my first OB appointment, ultrasound measured me at 5w 6d with just a gestational sac noted; no yolk or fetal pole. My beta hcg came back at 8,000 so the doc says it might not be a viable pregnancy. (She was hoping it would come back at closer to 5,000) I am having my levels re-drawn Friday in hopes that they double. Does anyone have any previous experience with this situation?