• What does this tool measure?
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• What do the results tell me?
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What does this tool measure?

This interactive tool estimates your peak fertility period, also known as your "fertile window." This is when you are most likely to get pregnant. Do not use this tool to prevent pregnancy.

To find your peak fertility period, the tool first calculates the day you are most likely to ovulate. This is the day an ovary releases an egg. In the tool, you will enter the typical length of your menstrual cycle, and you will click on the first day of your last menstrual period.

• To know how long your cycles are, track the number of days on a calendar for 2 or 3 months or cycles. Your menstrual cycle begins with the day your period starts and ends the day before your next period starts.
• If you do not know the number of days in your menstrual cycle, you can use 28 days. This is the average length of a menstrual cycle. But if your cycle is longer or shorter than that, or if it is not always the same length, this tool will not predict your fertile window very well.

This calculator is meant to give you a rough estimate. Women usually ovulate at day 15, but it's also normal to ovulate well before or after the 15-day mark.

For information about reading your body's signs to tell when you will ovulate, see Fertility Awareness.