1. What is a pregnancy calculator? Pregnancy Calculator is a tool for calculating your due date and gestational age. It calculates due date and gestational age based on the date of a woman\’s last menstrual period and the average menstrual cycle length. By entering this information, the pregnancy calculator can help expectant mothers understand the progress of their pregnancy and plan for future preparations. 2. How to use the pregnancy calculator? Using the pregnancy calculator is easy. First, you need to know the first day of your last menstrual period. Then, you need to enter the length of your usual menstrual cycle (i.e. the time between the start of one period and the start of the next).