This happened to me for the first time in a long time at the doctors office the other day when I was asked my birthday. I just said, very casually, deadpan and matter of fact, “I have a stutter, sometimes I hesitate on words”. The guy was like, “oh, gotcha” almost looked embarrassed and quickly moved on in the conversation. I think this is the best way to approach this situation. Most people don’t know people who stutter and I think it never even crosses their mind when this happens.