It is easier for women with regular menstrual cycles to track their fertile days, that is, the timeframe when their chance of getting pregnant is at its highest. Ovulation usually takes place on day 14 of the cycle. In other words, 14 days after menstruation. The days prior to ovulation are considered fertile days as well.
In women with shorter or irregular menstrual cycles, the most accurate way to calculate their fertility window is to subtract 15 days to the day when the next menstrual period is expected to start.