HCG Slow - Small Sac

Hi guys - some background info. Hubby & I weren’t trying, but also not preventing, and ended up with a positive test on 9/24. Went to the doctor for a blood test & HCG was 2,840. Went back a week later & HCG was 17,000. We t back yesterday (again two weeks later) & HCG was 39,810 & dr said US showed a very small sac that they weren’t able to see anything. I’m going back in two weeks for another US, but gosh two weeks seems soooo long now. Dr said it could be I ovulated late since my cycles are irregular (34-42 days), or that it’s not a viable pregnancy. Has anyone else experienced this or know someone who has? I’m trying not to stress! TIA!