This Saturday I think I will be 27 weeks. Maybe. According to my ob I will be 26 weeks. I stopped taking birth control early may. I got my period the last week of the pills. In June, I had a very light period. Im almost positive my period was between the 10-14. But my docter is thinking is was the 15th. I have a period for 3 days on a 28 day cycle. On July 19th, the u/s said I was 4 weeks and 6 days. On August 10th the u/s was saying I was 8 weeks and 1 day. Now, my ultra sounds are saying I'm farther along then my ob and my belly is also measuring 30 cm and my ob thinks I'm 26 weeks. My due date according to my ob is march 22nd 2013. Is it possible I am farther along?

Dates of conception and due days are estimates, based on information you provide and from size of average fetus compared to your fetus at certain weeks.
They can easily be off a week and sometimes two.
So yes you may well be a week further along,

A secret with babies, they will come when they want to, no one tells them the due date