Calculating the exact date of conception can be challenging, as it depends on various factors such as the length of your menstrual cycle and the timing of ovulation. However, we can estimate the window of conception based on your due date. Assuming a typical pregnancy duration of 40 weeks, counting back 38 weeks from September 14th will give you an approximate range for conception. This means that your most likely time for conceiving was around December 22nd to December 28th. Keep in mind that this is just an estimation, and individual variations can occur. If you want a more accurate determination, consult with your healthcare provider who may consider additional factors like ultrasound measurements or other medical information to provide a more precise answer.