That adds up to about 40 weeks after the first day of your last period. And if that doesn't seem imprecise enough, all of this is based on the assumption that everyone has a 28-day menstrual cycle and that everyone ovulates on the 14th day of that cycle ( they don't ). Plus, there's the fact that doctors sometimes change your due date when taking ultrasound results, your irregular periods, or later testing into account. We also know that first pregnancies are usually longer than subsequent ones, yet due dates are all calculated the same way. If you, like Bey, are carrying twins, you'll probably give birth earlier ( usually around 37 weeks ), but your due date calculations will be the same.