Sac and no baby?? help.

Today I went in for my first visit.  Per my last cycle I should be 7 weeks.  
I've had off and on spotting for about a week or so.  
Today they did a vag ultrasound and saw the sac but no baby.  They measured it at 5 weeks so she (dr) said I could just be earlier than expected but the spotting concerns her.  So ordered hcg test and I go back Thur. I will get results Friday.  So that is a LONG time to wait and wonder.  Has any other moms had similar experience and had healthy babies.  
Any advice would be great