What is the normal duration of the menstrual cycle? How does the calendar contraceptive method work?

The normal duration of the menstrual cycle is 28 days but it can vary between different women or in different cycles of the similar woman.

In the calendar contraceptive method the date n-14 (n minus 14) is taken considering n the number of days of the normal menstrual cycle of the woman (normally n=28). The safety margin +3 or -3 refers to the days around n-14 that intercourse should be avoided to stop pregnancy. (This method is not exempt from failures. A doctor must always be consulted before relying on any contraceptive method.)