My lmp was march 24 i got a positive pregnancy test a week after i missed my next period. why is my edd is january 13? when it should be in december

EDD is Dec. 28 if...: If cycles are 28 days long, the EDD should be December 28 for a Last Menstrual Period that started March 24. If there was a "dating ultrasound" scan done between 8 and 11 weeks of pregnancy, that scan should be more accurate than counting from an LMP. When counting from an LMP, the EDD depends on the cycle length... meaning that a really long cycle length could push the EDD out to January 13.