It is possible that you ovulated later than you thought and that your dates are off a bit. I would just wait the two weeks, and if at that time there are still no babies, then you will have your answer.
I know the wait is awful..but at this point, its really all you can do.
ARe they doing bloodwork every other day to make sure your HCG level is rising?
Did you get your HCG levels? If they have dropped then you should be worried. Seeing the sacs is a good sign! The tech may mot have seen the babies because right now they are the size of a seed. or (human error) I think they should have given you an ultrasound to see the side of your uterus not the bottom.
If you haven't bled and your levels haven't dropped, don't stress you and your babies out.
Keep praying and Congratulations! :)