1. What is the pregnancy cycle? The pregnancy cycle refers to the period from the first day of a woman’s menstruation to the time before the next menstrual period. Usually it is 28 days, but it may be shorter or longer than 28 days. During this cycle, many changes occur in the female body, including processes such as follicle development, ovulation, fertilization, and embryo implantation. 2. How to calculate the pregnancy cycle 1. Understand your menstrual cycle: record the start and end time of your menstruation each month and calculate the average. 2. Calculate the ovulation day: Subtract 14 days from the average menstrual cycle to get the ovulation day.