Doctors and midwives start counting pregnancy from the first day of your last menstrual period (LMP) even though, of course, you weren’t pregnant then.
The first day of your LMP is typically about two weeks before you ovulated and conceived your baby. We count from there because most women don’t know exactly when they ovulated, but most of them do know when their last period started.
Your practitioner counts 280 days from the first day of your LMP to determine your due date. That’s exactly 40 weeks. Your due date is just an estimate, however. Only 5 percent of babies are born on their due date.