Calculating weeks

Hillary
So at my last ultrasound they said baby was measuring 15 weeks. 
So now I'm four months exactly. 
But when I count 16 weeks back on the cAlander it takes me to like two weeks before I even conceived ? 
Is that right ? It confises my husband and he is always saying he doesn't understand how I am that far along. I think he's trying to say I got pregnant from someone else or something