High HCG, No Fetus: Understanding Potential Causes & Next Steps

Question
Good Day,
My friend had an ultrasound and a blood test yesterday. Blood test showed her HCG levels were 13'000 and ultrasound showed no fetus, but only large sac. She didn't have any bleeding recently. Doctors in the hospital  didn't tell us exactly what could happen but seemed to be worried. She is having another US in 10 days. My husband, who is oncologyst told me that it can be some form of cancer in young adults (she is 21), my mom says that fetus was probably killed by some infection and I also read that it could be twins? I'm very confused and nervous now, what could happen? Thank you very much.

Answer
I think someone described this to me last week sometime. You did not mention when the first day of her last perod was, but can see a level of 13,000 at 4 weeks as well as up to 12 weeks. We do not see a fetus by sonogram until 6-7 weeks from the date of her last LMP. She is having a sonogram in 10 days. If there is no fetus and she is more than 7 weeks. It may be a blighted ovum. Her age has nothing to do with it. I would discuss it with your obstetrician after the sonogram.