These women would avoid sex or use a barrier contraceptive during their calculated fertile days. As women's cycles can change, women using the rhythm method should update their calculations on a monthly basis.
When used within the measures stated, the rhythm method prevents pregnancy in 91% of cases. However, many women make errors in the calculations or fail to update their calculations. This decreases the average effectiveness to between 75 and 87%.
Despite its relatively high failure rates, the rhythm method may be attractive to some couples who prefer a more natural form of birth control or who shun other contraceptive methods for religious reasons. The rhythm method is also free unlike other forms of birth control. It also doesn't carry a risk of side effects, but it does not protect from sexually transmitted diseases.
The rhythm method can also be used in the reverse by couples hoping to fall pregnant. Similar calculations can tell couples when they should have sex to increase their chances of conception.