My lmp was 7/12/202. i had a positive at home pregnancy test on 8/10/2020. based on an ultrasound test dated 8/26/2020 my aua was 6w3d+/-0w4d. when did i have sex leading to this pregnancy?

I answered your previous question and I stated that the conception time will be approximately 45 days + 3 days = 48 days, so the timing of that event will be 48 days + 1 to 3 days, i.e. 49 -51 days (the sperm can survive in your uterus and fertilize an egg in an average of 3 days).

As a guideline, if your cycles are regular at 28 days and length of period is average,& if your period began on 7/12.you would have ovulated on or about 7/26. The egg lasts one day. Sperm can be viable up to 5 days. I think pregnancy could have occurred from sex from 7/21 to 7/26 when you were fertile. Bear in mind this is just a guideline & not etched in stone. Your gyn can further advise you.