To accurately calculate your due date, you'll need to pinpoint the first day of your last menstrual period (LMP). Your due date is approximately 40 weeks later and can be loosely determined with this simple formula:
• Take the first day of your LMP
• Add seven days
• Subtract three months
So if your LMP was August 30, for example, your due date would be around June 6. If your LMP was December 1, your due date would be around September 8. But your doctor can give you a more precise due date after your first prenatal examination, which may include an ultrasound (measuring the size of your uterus helps your doctor make a more accurate estimate of your due date).
Getting an accurate idea of your due date is important to your doctor because it helps to monitor your baby's progress and interpret tests results. But don't get too attached to your exact original date. It's normal for due dates to shift by a few days or so as your pregnancy progresses -- especially if your menstrual cycle was irregular before you got pregnant -- which means your baby may be farther or less far along than you originally thought. And don't plan on automatically rushing to the hospital when the big D-day rolls around either: Only about 5 percent of babies arrive on their actual due dates.