Your due date is only an estimate of when the baby will come. That is why is it called an Estimated Due Date or EDD. Not very many women actually deliver on their due date.

And, your due date is based on the first day of your last period. Yours was March 3rd - so even if you conceived on March 29, the estimated due date would remain December 12th.

Good luck with your delivery!

Dec. 12 happens to be my due date calculated based on when I ovulated (LMP due date is Dec. 10), and I know I conceived on Mar. 20 (I had sex only once during my fertile phase). It's most likely you conceived Mar. 24......looking at my chart, I can see I ovulated on Mar. 21. It is unlikely you would have ovulated so soon after your period ended (though it can happen), especially if you tended to have cycles of 28 days or longer.

The dates given by u/s can vary a week or so in either direction.