Is it a baby or blighted ovum?

I had an unusual experience the other day.  I had found out I was pregnant two weeks ago, and based on my last period back in October it was estimated that I was 9 weeks pregnant.  I went in 3 days ago for my first OB visit. (by the way this is my 4th pregnancy) and the MD did an ultrasound.  All that could be picked up was a yolk sac.  I have never been more suprised in my life. The MD told me that I was either in very early pregnancy and it is too early to see anything or it could be a blighted ovum and I would eventually miscarry.  I had a Quant level drawn on Friday and have to have another one done tomorrow.  I am wondering though my last period in October I had still been on birth control and I stopped using it after that period.  I did not have a period in November at all.  Is it posible that I am off on how far along I am and really in early pregnancy?  Has anyone else had a similar experience?  I am trying not to get my hopes up.  I did not even know I wanted the baby until I was only able to see the yolk sac.  Then it felt like a major let down. I have not had any s/s of miscarriage.  With my last pregnancy I was off by about 4 weeks.  I was actually 4 weeks ahead of what I thought I was.  Any insight would be a great help!

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Same things happened to me with my first. We went in for a u/s and all that could be seen was a yolk sac. Went back two weeks later and there was a baby. I was off on my dates.