You can also ask your doctor. He can tell you by one of those circle calender things they have to figure out how far along you are. They just go by your due date. And it says approx. when you ovulated.
Also, if you know when the first day of your last period was. You usually ovulate 7-10 days after the first day of your last period. If you don't remember your doctor should have it on file. He would have had to have an approx. date to figure out how far along you are unless he did that by ultra sound. Even then they can figure that out for you.
If your due date is September 11th , conception would have been on December 19. Give or take, sometimes due dates are wrong. I calculate my ovulate days through this Ovulation Calculator, and I gain instant result
According to my humble opinion if your's due date is 11th September, then add 40 weeks for your pregnant date.
To track your pregnancy through your due date ,here are pregnancy week by week tracking by using pregnancy calculator.
Hope that helped!
The due date is calculated by adding a week in the first day of your last menstrual period plus 9 months.
If you due date is 11 September, you might have got pregnant in the first week of January.
For more details, check out the answer of another blurter by clicking here