Question

At my very 1st OB appt, my doc did any ultrasound and measured me at 6w1d and gave me a due date of 7/5/15. He told me that he would do a second ultrasound at my second appointment for better dating purposes since at 6w1d, the baby is so tiny. At my second appt, which was at 11w1d based on his due date from the 1st ultrasound, and I measured at 12w0d. The official ultrasound report read a new due date of 6/29/15. Today I went to an appt, expecting for them to have me at 15w4d, but it turns out that they're still using the original date of 7/5/15 and had me at 14w5d. Yet when my doctor felt my uterus, he said I was right about at 16w size. So if I'm measuring considerably bigger, why is he still using a different date???