Calendar method
This method of contraception. can be used if your menstrual cycle is regular.
The conception is most likely to occur if the couple has sex two or three days before of after ovulation. If you know exactly the day of ovulation, you can use some other methods these days, and other days you do not have to protect yourself from unwilling pregnancy at all.
If the duration of your menstrual cycle is stable, ovulation usually falls on its middle. It means that if, for example, the duration of your menstrual cycle is 30 days (from the first day of one period till the last day of the other one) ovulation is most likely to fall on the fifteenth day. In this case you should use some methods of contraception five days before and five days after ovulation.
This method of contraception is quite reasonable from biological point of view, and if female organism worked as accurate as Swiss watch, everything would be wonderful. But unfortunately, it is rather difficult to predict exactly the first day of the period. If the difference between a womans menstrual cycles is more than a week, there are only a few days in the month when she cannot get pregnant.