safe period method
In a regular, normal ovulatory menstrual cycle, ovulation takes place on 14th (12-16 days:range) days before next menstrual cycle.
once the egg is realesed, it is viable only for 24hrs. if during this time, fertilization of egg and sperm fails to take place, it can not occur in the remaining days of cycle.
hence, the remaining days of cycle are safe from contraception point of view.
abstinence from coitus for 12 days in the middle of cycle. i.e. from 8-18 days of cycle.