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Using the rhythm method to promote fertility doesn't pose any risks.

Likewise, using the rhythm method for birth control doesn't pose any direct risks — but it doesn't offer protection from sexually transmitted infections. In addition, the risk of unintended pregnancy with the rhythm method is higher than with some other methods of birth control. Although effectiveness varies, in the first year of typical use an estimated 13 out of 100 women practicing the rhythm method for birth control will get pregnant.

Tracking your menstrual history doesn't require special preparation. However, if you want to use the rhythm method for birth control, consult your health care provider first if:

• You recently had your first period, gave birth, or stopped taking birth control pills or other hormonal contraceptives
• You're breast-feeding
• You're approaching menopause
• You have irregular menstrual cycles

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, the first day of your cycle is the first day of menstrual bleeding and 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, the first day of your cycle is the first day of menstrual bleeding and 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.