How to Calculate Pregnancy Time In order to calculate the length of pregnancy, we need to know the time of conception. The time of conception usually refers to the first day of a woman\’s last menstrual period. If your periods are regular, you can calculate your pregnancy period by counting your cycles to predict when you\’ll be pregnant. Generally, the pregnancy period is calculated based on the number of weeks after conception, with a week being 7 days. For example, 40 weeks pregnant means you have been pregnant for 280 days. In reality, the timing of pregnancy can be a little more complicated than the menstrual cycle. If your periods are irregular, or you choose artificial insemination after failing to conceive during your natural cycle, you will need to