Pregnancy... Is measured from the first day of your last period. Even though you may know the exact date you conceived, by convention, the medical community starts counting the beginning of your pregnancy as the first day of your last period. This is because people don't always have 28 day cycles, so it is difficult to know exactly when one has ovulated or conceived.
40 weeks. We start dating pregnancy from the first day of the last menstrual period, since that is usually easier to recall than the date of conception. The date we expect the baby to be born is 40 +/- 2 weeks from that date. This can be confusing when you try to convert to months because it means pregnancy lasts 10 months!