This method uses the calendar to count days between cycles and anticipate when your fertile days will be.  Most women with regular periods have a 26-32 day cycle, though your cycle could be as short as 23 days, or as long as 35 days. A wide range of possible cycle-lengths are still normal. The first day is the beginning of one period; the last day is the beginning of the next period. Remember, though, that your period may vary slightly from month to month. You could be on a 28-day cycle for one or two months, and then shift slightly the next month. This is also normal. Using a regular calendar, circle the first day of each cycle (the first day of your period).  Count the number of days between each cycle (include the first day when you count). Keep a running total of the number of days in each cycle. If you find that all of your cycles are shorter than 27 days, don’t use this method as it will give inaccurate results. Find the shortest cycle among all those you’ve tracked, and subtract 18 from that number of days.  Write down the resulting number. Then locate day one of your current cycle on the calendar. Starting at day one of your current cycle, use the number you wrote down to count forward that number of days. Mark the resulting day with an X. The day you’ve marked with an X is your first fertile day (not your ovulation day). Find the longest cycle among all those you’ve tracked, and subtract 11 from that number of days.  Write down the resulting number. Locate day one of your current cycle on the calendar. Starting at day one of your current cycle, use the number you wrote down to count forward that number of days. Mark the resulting day with an X. The day you marked with an X is your last fertile day and should be your day of ovulation. This method requires careful and consistent record-keeping and can thus be prone to human error.  Because your monthly cycles may shift, it’s difficult to precisely time your ovulation with this method. This method is best used in conjunction with other...

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Write a summary of the previous text in Chinese:
了解你的生理周期。 记录下至少8个生理周期。 预测你易孕期的第一天。 预测你可以受孕的最后一天。 了解这种方法的局限性。