My lmp was march 24th with a due date of december 30 but got changed to january 13 after first sonogram but my ga on sonogram dates back april 15th.

U/S dates for G/A: Early sonograms in the first trimester are dependable for fixing an approximate date for the term of pregnancy. Ultrasound done after 4-5 months may vary in the dates predicted due to different growth patterns because of genetic variation. A discrepancy of only two weeks or so is common, but later physical exams will be a better gauge of your due date.