When you find that you are pregnant, one of the first things you will do is ask about your pregnancy due date. Though these are pretty accurate, they are not as accurate as you might think. You can never depend on your baby arriving on the due date, and many women find themselves frustrated when their pregnancies go beyond the special day. If more people understood how the due date was calculated, they would understand why sometimes a pregnancy goes longer than what the doctors expect. Most doctors still use a wheel to calculate pregnancy due date. This is fine if you are a woman that has a perfect 28 day menstrual cycle. Perhaps perfect is the wrong word.