Thanks for using the forum.
Your due date is usually calculated starting from the first day of your last menstrual period (LMP). This is where the 40 week time span of pregnancy starts. Ovulation usually occurs at about 2 weeks after that day. Conception would have occurred 12-24 hours after ovulation. Therefore, conception most likely occurred 13-17 days after the start of your last period.
Congratulations on your pregnancy. I hope that this helps.