Conception date help..?

[KeenMoose]

If I got a first positive test on the 18th July and (continued each day to get darker lines)is it scientifically possible this could have been from sex on the 12th July? Or is it that impossible and would have been from earlier?

unlikely I'm afraid... at the earliest implantation can happen 6 days after fertilisation but even then it would take a couple days for the hormones to be high enough to be detected. From sex on the 12th, your 👀 my atleast 4 days so really you'd be looking at 22nd+ for that day