My due date is on 13july2018 so does that mean i was conceive on 13october2017 and its 39weeks counted or when did i had sex that lead to pregnancy?

38 weeks earlier: Remember, all dates are estimates when looking at pregnancy dating for a natural pregnancy. From the due date, just count back 38 weeks to get to the estimated conception date (when sperm met egg). Sex likely would have been 1-4 days before the conception date, but sometimes 0-7 days before the conception date. The actual conception date rarely matters in real life.