Periodical is a free application that calculates the fertile days according to Knaus-Ogino and can track menstrual symptoms. The data can be backed up on a memory card or "internal" memory, if present.

It is recommended to use this application to avoid pregnancy, but it is not the only accurate method to do so. There are more accurate methods, like measuring the basal body temperature.

The application has a warning:

The data calculated by the application should not be used to avoid pregnancy. It is not recommended to use the data to avoid pregnancy, and there are more accurate methods, like measuring the basal body temperature.