Sac but no baby: miscarriage?
Having a sac but no baby could mean a few different things.
It is possible that you are not as far along as you think you are. Your doctor might recommend wait another week and coming in for a repeat ultrasound to see if there is any change. Your doctor might also order some blood work to test your level of HCG.
A sac with no baby could also be an indication of a blighted ovum. A blighted ovum means that a fertilized egg implanted in the uterus but an embryo never formed from the group of cells. This is a form of miscarriage. Your doctor might confirm this with an ultrasound and/or blood test.