Unfortunately the due date is 40 weeks from the date of the first day of the last period - the date is usually estimated these days from the scan at 12 weeks acccording to the average size of a baby at that age - so you can only get a best guess as to the date of conception as redcrx says.
40 weeks represents the average length of a pregnancy - usually babies are brought out somehow if you dont have them at 40 weeks - but 36 weeks on is considered a normal pregnancy.