If my transvaginal ultrasound dated me on the 20th september at 7 weeks & 2 days - when did i conceive and how accurate are they?

Last period : The ultrasound done at that time is fairly accurate. Therefore, you got pregnant some time between the 20 - 23 july.

Ultrasound dating: It depends on the timing of ultrasound. First trimester ultrasound is accurate within 5-7 days. 2nd trimester ultrasound has accuracy of +/_ 2 weeks. Lmp dating also assumes a 28 day cycle. There is no test that can date conception more accurately than a 1 week window.