Measuring small on u/s

Hi everyone. I just went to the doctor the other day as I am not positive on my dates, however I believe that my LMP was somewhere between November 23 and december 01 2011. I have had some minor brown spotting for about the past 4 days and yesterday I went for an u/s hoping to see a heartbeat as I should be approx 6-7 weeks... While I was there they measured the gestational sac as being only just under 5 weeks... I am worried that I am having a miscarriage. My doctor is optimistic and said that he has no reason to think that I am so far, and that I should have another u/s done in two weeks to see if the baby is growing as it should to rule out a miscarriage.
Another reason I am concerned is because I went to have a doctors pregnancy teat done on December 29 2011 because I felt as though I was pretty late (not rememberi my LMP) and the ruine test was negative, however the blood test was positive. Then two days later I took the clear blue digital test and it said that I was 4-5 week pregnant... But that was almost two weeks ago now! And I was just told by my doctor that the u/s is showing just under 5!
Please help with any thoughts, or if you or someone you know has had a similar situation and has gone on to have a healthy pregnancy.

Thanks so much! Really looking forward to seeing what everyone has to say!  

[stacey10]

when you take a digital and it comes up with weeks all it is doing is measuring the amount of hcg in your urine and estimating how far along you are, they are using averages, you may have a higher level of hcg in your urine, eg be above the average but still within normal range so it gave you an estimate of how many weeks due to those measurements, one way to find out how your pregnancy is progressing is to have a couple of blood tests done 48 hrs apart to measure your hcg levels and if the are doubling or rising by at least 60% in that time then you have a viable pregnancy, otherwise I would try not to worry (hard I know) and wait for your follow up u/s