In the calendar or fertility awareness method of birth control, women tracks their menstrual cycles to determine on which days they are fertile and therefore at the highest risk for pregnancy. The calendar method is generally safe but carries a higher risk of pregnancy than hormonal or barrier methods of birth control, notes Planned Parenthood.
Planned Parenthood recommends that a woman tracks her menstruation for at least eight full cycles before adopting the calendar method as her primary method of birth control. This allows her to determine if her cycles are regular and how long they tend to last. Women with cycles that are consistently shorter than 26 days should not use the calendar method. Using the calendar method for birth control, a woman may be able to accurately determine the days in her cycle when she is the most fertile. On these days, she should avoid unprotected vaginal sex.
The standard days calendar may be used by women whose cycle is between 26 and 32 days long, according to the Family Planning Handbook. By this calendar, the first day of the woman's period is counted as day one. Days eight through 19 are the most fertile, and unprotected sex should be avoided. In the days following day 19 and up until the beginning of the woman's period, unprotected vaginal sex is unlikely to cause pregnancy.