What does this tool help you learn?

This 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.

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 don't 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 isn't always the same length, this tool will not predict your fertile window very well.

Fritz MA, Speroff L (2011). "Clinical Gynecologic Endocrinology and Infertility," 8th ed., Philadelphia: Lippincott Williams and Wilkins.

What do the results tell you?

Your "fertile window" is up to 6 days long, once a month. It includes:

• The day you ovulate. This is when you have the best chance of becoming pregnant. (A human egg usually lives for only 12 to 24 hours after ovulation. This is why you are not likely to get pregnant by having sex a day after you ovulate.)
• The 5 days before ovulation. This is because sperm can live in a woman's body for 3 to 5 days after sex. When an egg is released, one of these sperm is ready to fertilize it.

If you want to become pregnant, try to have sex every day or every other day from your first fertile day to your last fertile day.

What's next?

If your periods are irregular, this calculator isn't a good way to predict your ovulation dates. Do not use this tool to prevent pregnancy.

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