First, record the length of six to 12 of your menstrual cycles, from the first day of your period to the first day of your next period, according to the Mayo Clinic . Then, you take the length of your shortest menstrual cycle, and subtract 18 from the number of days to find the first fertile day of your cycle. Next, take the length of your longest menstrual cycle and subtract 11 from the number of days to determine the last fertile day of your cycle. Now that you have this information, you have to plan sex around when you're most fertile — either going hard during those days, if you're trying for a baby, or keeping pants firmly in place if not. To ensure accuracy, it's best to keep updating your calculations every month.