Why when the doc calculate a due date, do they subtract two weeks out of 40 weeks?

Doc's don't subtract. Doctors don't subtract 2 weeks from the 40 weeks. It is somewhat confusing, but doctors use the 38 weeks number only when adding 38 weeks to the date of conception to get the due date. For almost all pregnant women, the mathematical calculations are not important after a doctor has accurately set a due date.