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?

According to a quick google, your body doesn't produce enough hcg for a test to be positive until 10-14 days after conception. So I'd assume it would have to been from an earlier date

The earliest you can get a positive HPT is 8 dpo.

An egg can be fertilised up to 24 hours after being released so this would mean you caught the egg right at the end of the opportunity window on the 12th meaning you were 7 dpo when you tested positive.

I think it's unlikely.