Do blighted ovulum hcg double like it should ?

I went in for a 6 weeks scan and they said they THiNK they saw a yolk sac but not sure since I’m early and I have a tilted uterus . Then they ordered my hcg it was 7,698 and now it went up to 19,899 . Could I just be really earlier than I thought ? Or do blighted ovums keep doubling hcg anyways ?