Is Normal Pregnancy Possible When HCG Level In Low?

I am 6 weeks pregnant based on my LMP (4th baby). I had pain at 4 wks 3 days and was assessed at hospital for a possible ectopic pregnancy. My hCG level at that time was 127. Two and a bit days later my level was 250. I had an ultrasound at 5 wks 4 days which showed a small irregular shaped empty gestational sac and my hCG level had only gone up to 310. My hCG level today (6 weeks) was only 407 and I was told it was not a viable pregnancy (most likely ectopic). I had another ultrasound this afternoon though that showed a gestational sac with yolk sac and possible heart beat! From what I have read, this seems impossible with my hCG levels being so low. I'm not booked in to see my Dr for another two days and am quite anxious.