A due date is frequently a guess based on your latest menstrual cycle. The estimate assumes you have a typical 28-day cycle and that ovulation occurs 14 days after your last menstruation. Counting 280 days (40 weeks) from the first day of the last menstrual cycle yields the due date. However, since women's cycles vary in length and timing of events, the actual due date may be earlier or later than the estimated date. It is not possible to precisely determine the date a baby will be born from the first day of the previous menstrual period. The earliest a due date could possibly be known with certainty is 36 weeks into the pregnancy. After this point, there are only estimates of the due date. Some factors that may affect the due date include how far along you are in your pregnancy, how many pregnancies you have had previously, and any medical conditions you may have. A due date can also change during a single pregnancy. As the fetus grows, it takes more space inside the uterus, which causes the pregnant woman's body to adjust the date of her next period so that it does not coincide with the time of birth. Due dates are used by doctors to plan for deliveries. They make sure that all necessary tests are done before the due date and prepare hospital rooms for babies who are coming soon. Knowing that one has to wait until some future date to meet a loved one or pick up a package cannot be predicted until that date comes.