Make life simple by using a pregnancy calculator. It is a simple calculator that will help you to know your due date of delivery, conception, and other pregnancy events. Why must you use this? Because it could prepare you for what is to come in the future, it predicts events of your pregnancy that would finally end with the delivery of a new born. The estimate is done based on the last menstrual period and the estimated date of conception. The calculator adds 40 weeks to the approximate first day of your last menstrual period to the day of ovulation.