Rhythm method

rhythm method
She needs her menstrual cycles registration for a period of at least 6 months.

The first day of each menstrual cycle is the day you start your period (Day 1). Then days are numbered below (day 2, day 3, etc.) until the last day before the next menstruation. A menstrual cycle is the time from the first day of menstruation / Day 1 of the menstrual cycle and the first day of the next cycle, can vary between 25 and 35 days.

Shortest menstrual cycle minus figure 18, we obtain the number of days that begins ovulating.
Longest menstrual cycle minus figure 11, is obtained as the day ends fertile period.

example:
A woman has menstrual cycles lasting between 26 and 31 days
26-18 = 8. From day 8 of the menstrual cycle to avoid any sexual contact, so it can be sexual until day 7, inclusive.
31-11 = 20. Can have sex after 20-day menstrual cycle (period of sobriety between days 8 and 20 of the menstrual cycle).