Period Tracker helps you determine your fertile days of the month, so you can either achieve or avoid pregnancy. The calendar uses the date of your last period and shows the next period date, the best days to conceive a girl or a boy, your ovulation day and when you should avoid pregnancy. To generate your ovulation calendar you should provide the following details: - Date of the last period (on the calendar on the main tab). - Average length of the periods (on Settings tab). - Length of the Luteal Phase (on Settings tab or use default value). The application works best for women with regular menstrual cycles, the average cycle length is also a valid way to predict when the ovulation should happen.