The rhythm method, also called the calendar method or the calendar rhythm method, is a form of natural family planning.

To use the rhythm method, you track your menstrual history to predict when you'll ovulate. This helps you determine when you're most likely to conceive.

If you're hoping to get pregnant, you can use the rhythm method to determine the best days to have sex. Similarly, if you're hoping to avoid pregnancy, you can use the rhythm method to determine which days to avoid unprotected sex.

Using the rhythm method for birth control requires careful record keeping and diligence. If you don't want to conceive, you and your partner must avoid having sex or use a barrier method of contraception during your fertile days each month.

The rhythm method can be used as a way to promote fertility or as a method of contraception, by helping you gauge the best days to have or avoid unprotected sex. Tracking your menstrual history for either fertility or contraception is inexpensive and doesn't have any side effects. Some women choose to use the rhythm method for religious reasons.

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 somewhat higher than with other methods of birth control. Although effectiveness varies, in the first year of typical use an estimated 13 to 25 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

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.

Dec. 17, 2011