What you can expectBy Mayo Clinic staff
To use the rhythm method:
- Record the length of 6 to 12 of your menstrual cycles. Using a calendar, write down the number of days in each menstrual cycle — counting from the first day of your period to the first day of your next period.
- Determine the length of your shortest menstrual cycle. Subtract 18 from the total number of days in your shortest cycle. This number represents the first fertile day of your cycle. For example, if your shortest cycle is 26 days long, subtract 18 from 26 — which equals 8. In this example, where the first day of your cycle is the first day of menstrual bleeding, the eighth day of your cycle is the first fertile day.
- Determine the length of your longest menstrual cycle. Subtract 11 from the total number of days in your longest cycle. This number represents the last fertile day of your cycle. For example, if your longest cycle is 32 days long, subtract 11 from 32 — which equals 21. In this example, where the first day of your cycle is the first day of menstrual bleeding, the 21st day of your cycle is the last fertile day.
- Plan sex carefully during fertile days. If you're hoping to avoid pregnancy, unprotected sex is off-limits during your fertile days — every month. On the other hand, if you're hoping to get pregnant, have sex regularly during your fertile days.
- Update your calculations every month. Continue recording the length of your menstrual cycles to make sure you're properly determining your fertile days.
Keep in mind that many factors can affect the exact timing of ovulation, including illness and stress. Using the rhythm method to predict ovulation can be inaccurate, especially if your cycle is irregular.
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