How to Calculate Menstrual Cycle

Knowing how to calculate a menstrual cycle is vital to any menstruating woman. The menstrual cycle is the amount of time it takes for a woman's body to develop and shed the inner lining of the uterus. Menstruation is the actual process of shedding the lining.

Menstrual Cycle
A woman's menstrual cycle occurs about every 28 to 30 days. Menstruation can last anywhere from a few days to an entire week. The exact menstrual cycle and menstruation time varies from woman to woman. Some women have regular menstrual cycles that occur a certain number of days apart, but others have an irregular menstruation cycle in which the time between menstruations varies wildly. Menstruation may occur after 24 days one month and after 31 days the next month.

Calculate a Regular Menstrual Cycle
Calculating your menstrual cycle is extremely easy and only requires a calendar. Mark a calendar on the first day of your period. Wait one month and mark the calendar the first day you get your period again. If you have a regular menstrual cycle, just count the days between the first day of your period in one month and the first day of your period in the second month. This number is how many days your menstrual cycle takes.

Calculate an Irregular Menstrual Cycle
If you have an irregular menstrual cycle, you may need to track your menstruation for a year or more to find a true pattern. If your menstruation is highly irregular, you may never be able to calculate anything more than a rough estimate of your cycle. To calculate your menstrual cycle, mark a calendar on the first day of your period, each month, for 12 months. You also need to mark how long your period lasts each month.

  1. Count days. Once you have marked the first day of your period each month for 12 months, go back and make a list of the days between menstruation each month. For example, count the days between the first day of your period in January and the first day of your period in February. Next, count the days between the first day of your period in February and the first day of your period in March.
  2. Look for patterns. Look for patterns in your menstruation cycle. Maybe your cycle is just slightly irregular, such as a repeating pattern of 24 days between menstruation one month and 30 days the next. Check the length of your menstruation each month for patterns too. For example, every time your menstruation lasted for five days, your next menstrual cycle didn't come for 32 days. These patterns will help you predict menstruation for the coming year.

The ability to calculate your menstrual cycle has many practical applications. It helps tell you when your body is the most fertile, which can be important for women who do or do not want to get pregnant. It lets you predict the timing of any premenstrual syndromes so that you can plan accordingly. For example, if you know you bloat the day before your period, you can reduce your salt intake during that time to help fight the bloating. It also lets you know the best times to plan special events, such as a scuba diving trip.

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