This involves recording your menstrual cycle on a calendar for eight to 12 months. The first day of your period is Day 1. Circle Day 1 on the calendar.
The length of your cycle may vary from month to month. So write down the total number of days it lasts each time. Using this record, you can find the days you are most fertile in the months ahead:
1. To find out the first day when you are most fertile, subtract 18 from the total number of days in your shortest cycle. Take this new number and count ahead that many days from the first day of your next period.
Draw an X through this date on your calendar. The X marks the first day you're likely to be fertile.
2. To find out the last day when you are most fertile, subtract 11 from the total number of days in your longest cycle. Take this new number and count ahead that many days from the first day of your next period.
Draw an X through this date on your calendar. The time between the two Xs is your most fertile window.
This method always should be used along with other fertility awareness methods, especially if your cycles are not always the same length.
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