Our formula for calculating your due date is based on the first day of your last menstrual period (LMP), and is the most common formula currently used by ob-gyns and midwives. You may also calculate by the exact date of conception, if you happen to know it, but because this circumstance is understandably rare, most due date calculation is based on the LMP. Here is what you may find surprising about this formula (and about the first few weeks of our Weekly Views): The first two weeks counted as part of the pregnancy usually actually precede ovulation and fertilization, which usually takes place in Week 3. That is how the convention works. Your due date is an estimated date, based on 40 weeks from the date of your LMP, since women commonly give birth between Weeks 38 and 42. If your ob-gyn or midwife alters your projected due date based on later developmental observation (such as that yielded by a sonogram), please go to the Settings & Profile page to enter the new due date and keep your personalized information up-to-date.