When do you think my due date is?

The dr. says im due the 7th of January. But it does not match up with my last period.. my last period was the 11th of March which would make my due date the 16th of december. when i had my Sonogram at "20 weeks" [according to them] my son measured above average of a 20 week baby, and a bit heavier than average of a 20 week baby.. more like the due date of the 16th.. what do you guys think what it is? the 16th, or the 7th.. it's quite a big differance.