there is no real way to know. doctors do a guesstimate on when you became pregnant and count the first day of your last menstrual period as day one of pregnancy and u/s can be off by up to 2 weeks.
if it's a question of paternity....get a dna test.
It would have Most likely happened on the 20th as that is closer to Your middle mark however it could have been the 5th anything is possible.
There are many websites You can join for weekly updates on how your baby is growing, symptoms to look out for, what is normal and what is not in pregnancy as well as a calender which will tell you your rough date of conception if you time in those dates.
Goodluck