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Rhythm method for natural family planning

Medically reviewed on January 6, 2018

Overview

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 persistence. 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.

Why it's done

The rhythm method can be used as a way to promote fertility or as a method of contraception, by helping you determine the best days to have or avoid unprotected sex. Some women choose to use the rhythm method if a complex medical history limits traditional birth control options, or for religious reasons.

Risks

The rhythm method is an inexpensive and safe way to help you chart your fertility — the time of month when you're most likely to be able to get pregnant.

Using the rhythm method as a form of birth control doesn't pose any direct risks. However, it's considered one of the least effective forms of birth control. How well the rhythm method works varies between couples. In general, as many as 24 out of 100 women who use natural family planning for birth control become pregnant the first year. Also, the rhythm method doesn't protect you from sexually transmitted infections.

How you prepare

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

  • You recently had your first period
  • You just had a baby
  • You've recently stopped taking birth control pills or other hormone contraceptives
  • You're breast-feeding
  • You're approaching menopause
  • You have irregular menstrual cycles

What you can expect

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, including medications, stress and illness, can affect the exact timing of ovulation. Using the rhythm method to predict ovulation can be inaccurate, especially if your cycle is irregular.

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