Actually the prevention or postponement of pregnancy by using the calendar system of family planning or periodic abstinence have the highest failure compared contraceptive methods other.
Why? Because even if you have regular menstrual cycles, often occurs inaccuracies in determining your fertile period. Thus, it is not advisable to use this method if you really do not want the pregnancy.
In the meantime, barrier methods such as condoms have success rates higher to prevent pregnancy, that is about 80-95 percent. For additional information, no single method of contraception can provide 100 percent guaranteed to prevent pregnancy.
Each method has a number of success and failure of different. Thus, depending on your choice. Choose a convenient means of contraception for you.