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Clinical Reference Systems: Pediatric Advisor 10.0

Natural Family Planning


Natural family planning is a way to practice birth control without using drugs or contraceptive devices. Two methods of natural family planning are the rhythm method (also called the calendar method) and the withdrawal method. (The withdrawal method is not recommended because it frequently results in pregnancy.)


  1. Your menstrual cycle

    During each menstrual cycle, two events occur: (1) you have your period (menstruation) and (2) you release an egg from one of your ovaries (ovulation). When you are ovulating you are fertile, which means you have a very high risk of getting pregnant.

    The first day of your period is the first day of your menstrual cycle. The day before your next period begins is the last day of your menstrual cycle. The average menstrual cycle is 21 to 35 days long.

    If you want to use the rhythm method for birth control, you must understand how your body works, keep a very accurate record of your menstrual cycle, and do some calculations. Your goal is to determine how long your menstrual cycles are and when you ovulate.

  2. Keeping a calendar

    On a calendar mark the days your periods start. From this information you can calculate the number of days in your menstrual cycle. For example, if you started your period on December 10, December 10 was the first day of your menstrual cycle. If your next period began January 7, the length of your menstrual cycle was 28 days (22 days in December and 6 days in January). Now, you begin again, with January 7 as day 1 of your next cycle.

    Menstrual cycles often vary somewhat in length from month to month. For this reason you need to keep a record of your menstrual cycle for 3 to 6 months. You will then have a pretty good idea of how long your shortest and longest cycles are. You will use this information to calculate the days when you are most likely to ovulate and get pregnant if you have sexual intercourse.

  3. When NOT to have sex

    Once you know the lengths of your shortest and longest menstrual cycles you can figure out when you have a high risk of getting pregnant. You do this by subtracting 18 days from your shortest cycle and 11 days from your longest cycle. From the results of these calculations you will know when you are most fertile. The result of the first calculation is the day you should stop having sexual intercourse. The result of the second calculation is the day when you can start having sex again.

    For example, let's say your shortest cycle in 8 months was 25 days and your longest cycle was 30 days. Subtract 18 from your shortest cycle and 11 from your longest cycle. Here's how the calculations look:

             25 (shortest cycle)      30 (longest cycle) 
            -18                      -11 
            ---                      ---
              7                       19 

    These calculations mean that from the 7th to the 19th days of your cycle you should NOT have sex. You will ovulate sometime during these days in your cycle and have a high risk of getting pregnant. Remember that the first day of your period is "day 1" of your menstrual cycle. So in this example you are at high risk of getting pregnant 7 days after the start of your period and for the next 12 days. For example, if your next period starts on February 5 (day 1), then you should not have sex from February 11 (day 7) to February 23 (day 19).

  4. Advantages of the rhythm method
    • This method helps you calculate which days of the month you are most likely to get pregnant.
    • This method helps you know more about your menstrual cycles and your body.
    • You may safely and inexpensively prevent pregnancy if you follow the rhythm method properly.
    • This method is accepted by most religions that oppose other contraceptive methods.

  5. Disadvantages of the rhythm method
    • If you use this method of birth control, you can have sex only during certain days of your menstrual cycle. Because you cannot have sex whenever you want to this method requires a lot of self-control by you and your partner.
    • You must keep detailed records of many (8) menstrual cycles before you use this method for birth control. You must continue to keep track of your cycles while using this method.
    • If your periods (cycles) are irregular, you cannot use this form of birth control because you cannot safely calculate when you are fertile.
    • This method is only 70 percent reliable for preventing pregnancy, even in women with predictable cycles. This means 30 out of 100 couples (or 3 out of 10) will have pregnancies if they rely on this method for a year.
    • You are not protected against sexually transmitted diseases (STDs). To protect yourself against STDs, you must use a condom every time you have sexual intercourse.


We do NOT recommend withdrawal as a method of birth control.

Many people believe that if a man and a woman are having sexual intercourse and the man pulls his penis out of a woman's vagina before he ejaculates, the woman will not get pregnant. (Ejaculation is the release of sperm.) However, this is NOT TRUE. There have been many, many unwanted pregnancies as a result of this approach to birth control. In reality, some sperm are released from the penis into the vagina BEFORE a man ejaculates. That is, by the time he pulls out, some sperm are already on their way to an egg.

The withdrawal method is not considered to be an effective form of natural birth control. Use of this method is strongly discouraged if you want to prevent pregnancy.


  • You have any questions or concerns.

Written by David W. Kaplan, M.D., and the staff of the Adolescent Medicine Center, The Children's Hospital, Denver, Colorado.
Copyright 1999 Clinical Reference Systems