I have an ob but they say my due date can change when i go back on the 10 of steptemeber when they due an ultrasound and all that prenatal update why?

More data: A due date can change because more and more measurements are made which allow a more accurate prediction.

Due date : The most accurate way to determine the due date is to calculate 38 weeks from the day of conception. In a woman with regular cycles of 28 days that would be 40 weeks from the first day of last period. Early ultrasound is also accurate, especially to confirm. Later changes to the due date are not more accurate and not advised.