Could My Wife Still Be Pregnant After Sex Despite Negative Tests?

Hi we had sex on the 26th August. And 30th my wife got her regular periods. She supposed to have next cycle after 28 days which is 26th September. But she missed it though after 26th August we never had sex. On the 2nd October she took a serum qualitative test for pregnancy and it was negative. Now the doctor gave duphaston for 5 days and today is the last day of the course. I am just wondering if she can be still pregnant from the 26th August sex we had. Please explain Thanks