Calculating ovulation or calculate your ovulation. When can I become pregnant? In your monthly cycle you have a short fertile period. In that fertile period you have approximately 2 days on which you have the greatest chance of becoming pregnant. With this ovulation calculator you can easily determine when your fertile period begins. The day on which you have the greatest chance of becoming pregnant can be determined with Sensitest ovulation tests. Specify the length of your cycle and the day on which your last menstruation started. If you do not know the cycle length, you choose the average of 28 days.