OBGYN told me that I was a week more than I really am....?

I am a little confused. I went to the obgyn last week for a check up, and during my last ultrasound, the doctor told me I was 11 weeks pregnant, but when the other doctor examed me, he told me that my uterus was measuring at 12 weeks... have this happened to you guys as well?? This is the first time for a obgyn to tell me this. By the way this is my third pregnancy ( gestational surrogate) and I have two kids of my own.