A woman is able to become pregnant (fertile) only during a certain part of her monthly cycle. That is just before and during ovulation. By learning when you ovulate, you can predict when you’re likely to be fertile. This calculator gives approximate ovulation dates and fertile times for women who have regular periods. (Regular periods mean having a period every 21 to 40 days.) Fertility charting, basal temperature tracking, and purchased ovulation test kits can also be used to help predict a woman’s fertile times. These are especially useful if a woman has irregular periods.