Conception

Is there any sort of possibility that an ultrasound can be a month off? Could I be measuring a month off? I had my first ultrasound scan very very early on and I want to make sure that my due date is accurate... 
My doctor told me I conceived in September. Is there any possibility I may have conceived a month prior?
I have had the same ultrasound date each visit. A day or two off but Ball park the same date(s)