The Calendar Method (Rhythm Method)
Research in the 1930s led to the development of the earliest of the natural family planning methods,the “rhythm” method, also known as the calendar method. The method was based on this finding, a woman’s fertile period can be identified.
This method has many limitations because of the wide variation in the length of menstrual cycles. Because a fairly regular menstrual cycles. Because a fairly regular menstrual cycle is essential to any reliable estimation of the time ovulation, the following women cannot depend on the calendar method: women with menstrual cycles shorter than 25 days, women with irregular menstrual cycles, women with menstrual cycles that vary in length by 8 days or more, postpartum women, women who are lactating and women in the perimenopause.
The calendar method can only predict the days in a menstrual cycle during which a woman is more likely to get pregnant. This prediction is based on the projected time of ovulation as determined by calendar calculations made from the history of the length of the last 8 to 12 cycles. A woman must keep a record of menstrual cycles to identify the longest and shortest cycles so that all possible fertile days may be projected. The calculations used today allow for a variation factor of +2 days around the approximately 14 days prior to the onset of the next menstrual period, 2 or 3 days for sperm survival, and 1 day for ovum survival, for a minimum total of 9 fertile days.
The woman subtracts 20 days from the length of her shortest cycle to determine her first predicted fertile day and 10 days from the length of her longer cycle to determine her last predicted fertile day. The couple then abstain from sexual intercourse during the projected fertile days to prevent conception.
Although this method may be selected by some couples in United States and is still used in many countries, it has been supplanted by modern natural family planning methods that have proved to be more effective and require fewer days of abstinence. The latter methods differ from the calendar (rhythm) method in that they are based on clinical indicators of hormonal changes and identify the times of fertility and infertility as they occur in each and every cycle. These biological signs can be observed and interpreted by women and their partners.
Source:
Helen Varney. Varney’s Midwifery. Jones & Bartlett Publishers, 1997.
