I was always told that the date given during your first trimester is the most accurate, I had a dating scan that told me that I was due on the 28th Sept but my LMP date was the 21st Sept. With every scan since they have said that the baby is measuring different to my date but that they will still work off the dating scan date.
Babies grow at different rates during the latter stages of pregnancy hence why they say to use your first trimester date.