With a blighted ovum Hun, the hcg will continue to rise as normal because the body doesn't realise the pregnancy is no longer viable. The placenta keeps growing and releasing the hormone. At least that's what my obstetrician told me. If you're unsure go to your doctor and ask for a referral for another scan so that you can feel 100% confident in your decision. I did this at 11 weeks & they will tell you for sure. At 9 weeks you should def see something.