I think the best answer to any of these questions is that every situation is completely different.  I stumbled across this post for the same reason any of you did...I had unanswered questions and I was left waiting.  The internet is a horrible place for a person like that.  But, just to share my situation, here's what happened:

I had an ultrasound at what I believed to be close to 7 weeks (based on a home ovulation test).  The doctor found and measured the baby at 6 weeks 1 day.  Much to my surprise, she also found a second baby.  She said she thought maybe she was seeing a heartbeat on one but could not definitively tell, so she asked me to come back in two weeks.  Two weeks?!  First the shock of doubles but then basically making me wait two weeks to see if one or both were viable?  It killed me, but I made an appointment 13 days later.  The wait was agonizing.  The first week I actually had more pregnancy symptoms like morning sickness and aversions.  Then the second week I started to feel better, which made me nervous.  

When I went to my follow-up ultrasound, the doctor immediately found two much larger babies, both with beating hearts.

At any rate, for those of you waiting for answers...I feel for you.  It can be the longest few days or weeks of waiting and I wish you well.  I really agree with those who seek second opinions after seeing some of these posts.  I can't believe how many doctors treat everything like a one-case study.  Every woman, baby, pregnancy, etc. is different and should be treated as just that and with care.  If you didn't get to see a heartbeat at 6 weeks give or take a few days, don't be discouraged just yet.