No heart beat measuring 8 weeks

I took a pregnancy test the last of 2010 and it was positive.  Went into the midwife to confirm pregnancy through urine sample the following week.  My first prenatal/ultra sound appointment was a few days ago.  The baby measured 8+ weeks and the sac a bit older, I was told there was no visible cardiac activity.  Supposedly I am somewhere around 12-14wks.
BUT I don't know when I got pregnant, I don't remember when my last period was.  I believe it was before Thanksgiving, but I am not sure.  Is it crazy to hope the dates are off and they just can't see the heartbeat yet?  I read that sometimes you can ovulate 11-21days after the end of your last period.
I think I am just trying to make it NOT SO.  I have not yet had any spotting or signs of miscarriage.  This is my 3rd pregnancy, and have 2 healthy kids where pregnancies were fine.  The last few weeks I have not 'felt" pregnant, but I have been busy and taking care of the other 2, I haven't really had time to feel pregnant.
Has anyone else had a reading of no heartbeat at this stage but the baby was alive?  I am supposed to go in each week to get blood drawn to test that the hormones are decreasing, what if we are all wrong.  Should I hope and ask for another ultra sound?

[AnnieBrooke]

I'm sorry you're going through this, I've been in the same place and it is not at all easy to take in this news when the doctor gives it.  

My suggestion is that you get the blood draws, and then if they are equivocal, ask for another ultrasound.  Certainly, ask for an ultrasound before you do anything like a D&C.  

Nobody on this site likes to say anything but good news, but do prepare yourself...if the embryo measures 8 + weeks in size and there is no heartbeat, then it is probable that the pregnancy has failed.  It would have to be a 5-week sized embryo for the lack of a heartbeat to be normal.  If this turns out to be correct, I'm so sorry.