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.

What do the results tell me?

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 is not a good way to predict your ovulation dates. Do not use this tool to prevent pregnancy.

For more information, see the topics Fertility Awareness, Pregnancy, and Fertility Problems. If you are looking for information on birth control, see the topic Birth Control.

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