Based on the information provided, it can be confusing to determine the exact duration of your pregnancy. The dating of a pregnancy is typically calculated from the first day of your last menstrual period (LMP), but in some cases, an ultrasound may be used to estimate gestational age. Ultrasound measurements are not always precise and can have a margin of error. It's important to remember that conception usually occurs about two weeks after your LMP.
In your case, if you had sex only on September 13th and 14th, it is unlikely that you conceived immediately after those dates. Sperm can survive in the female reproductive system for up to five days, so conception could have occurred within a few days after intercourse.
Considering this information, it is possible that you are around 3 and a half weeks pregnant based on your estimated conception date. However, it would be best to consult with your healthcare provider for a more accurate assessment. They can take into account additional factors such as your menstrual cycle length and provide further guidance regarding the duration of your pregnancy.
Remember that every individual's situation is unique, and it's essential to rely on professional medical advice for accurate information about your specific circumstances.