Calendar Method

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To prevent pregnancy, women can keep track of their menstrual cycles and abstain from unprotected vaginal intercourse when they are most likely to become pregnant. The methods you can use to do this are called fertility awareness-based methods (FAMs).

One way that women track their fertility patterns is called the calendar method.

Some women also track their fertility patterns when trying to get pregnant to know when they should have unprotected vaginal intercourse.

The Calendar Method

With the calendar method, you need to keep a record of the length of each menstrual cycle in order to determine when you are fertile. You can use an ordinary calendar. Circle day one of each cycle, which is the first day of your period. Count the total number of days in each cycle. Include the first day when you count. Do this for at least eight cycles (12 is better). 

Cycle Record

First Day of Period  Number of Days in Cycle
 Jan. 20  29
 Feb. 18  29
 Mar. 18  28
 Apr. 16  29
 May 12  26
 June 9  28
 July 9  30
 Aug. 5  27

Don't use the calendar method if all your cycles are shorter than 27 days.

Chart Your Calendar Pattern

To predict the first fertile day in your current cycle

  • Find the shortest cycle in your record.
  • Subtract 18 from the total number of days.
  • Count that number of days from day one of your current cycle, and mark that day with an X. Include day one when you count.
  • The day marked X is your first fertile day.

To predict the last fertile day in your current cycle

  • Find the longest cycle in your record.
  • Subtract 11 days from the total number of days.
  • Count that number of days from day one of your current cycle, and mark that day with an X. Include day one when you count.
  • The day marked X is the last fertile day.

Here is an example:

Predicting your first fertile day. If your shortest cycle is 26 days long, subtract 18 from 26. That leaves 8. If day one was the fourth day of the month, the day you will mark X will be the 11th. That's the first day you're likely to be fertile. So on that day, you should start abstaining from sex or start using a cervical cap, condom, diaphragm, or female condom.

Predicting your last fertile day. If your longest cycle is 30 days, subtract 11 from 30. That leaves 19. If day one was the fourth day of the month, the day you will mark X will be the 22nd. That's the last day you're likely to be fertile during your current cycle. So you may start to have unprotected vaginal intercourse after that day.

In this example, the 11th through the 22nd are unsafe days. All the others are safe days.

The Role of the Calendar Method

The calendar method can only predict what are most likely to be safe days. It is especially risky if your cycles are not always the same length. That's why it should always be used with other methods. Do not have unprotected intercourse on any day that the calendar method says is unsafe.

Effectiveness

Of 100 couples who use the calendar method correctly for one year, 9 will have a pregnancy.

This information is not meant as a substitute for professional advice.

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