For some of us there could be some confusion about how to calculate menstrual cycle duration and rhythms. Either because a woman is looking to get pregnant or because she is looking to make sure that an unintended pregnancy doesn’t happen, this is important to know how to calculate.

General pointers for how to calculate menstrual cycle

The menstrual cycle of the average woman is thought to be 28 days long. However divergence from this 28 day formula is far from unusual and is perfectly normal in a vast majority of cases.

It is perfectly normal for women to have a 23 day cycle as well as a 35 day cycle, and both can be fine so long as this is a predictable and regular pattern. Some women also have unpredictable or irregular periods, which may also be normal, but which should be checked by a doctor to be perfectly sure.

The most important pointer for how to calculate menstrual cycle is to start counting from the first day of a woman’s period until the last day before the next period, no matter what the length of a period is.

You can mark this day with a cross on your calendar or keep a small pocket calendar exclusively for this purpose. This will tell you at a glance, by the number of crosses, and where they are situated, the length of your cycle, its regularity, predictability and so on.

How to calculate menstrual cycle to avoid pregnancy

To avoid a pregnancy, it is important to know what a woman’s fertile days are so that extra care can be exercised on those days. However women can get pregnant even if they have sex several days before ovulation or even a day or two after.

Also it is difficult to pinpoint exactly which are the fertile days of a woman’s cycle in many cases, so if long term contraception (pill, IUD or similar) is not being used, women should use barrier protection each time they have sex if they want to avoid pregnancy.

How to calculate menstrual cycle for ovulation

For women who want to get pregnant, having sexual intercourse at the time of ovulation or as soon before or after as possible is of vital importance. Many women are quite regular and ovulation may predictably occur in the middle of a monthly cycle – usually about 14 days after the start of the menstrual period but many women ovulate earlier or later in their cycle. Also once a pregnancy occurs, the due date is calculated based on a woman’s LMP (last monthly period) which is also calculated as per the first day of the last period.

How to calculate menstrual cycle when periods are irregular

When a woman’s periods are irregular and unpredictable, calculating her menstrual cycle and ovulation date can be a challenge. However, plotting the dates of the start of a period on a calendar regularly could show if there is pattern emerging. If no pattern whatsoever emerges, a doctor should be consulted not only if a woman wants to get pregnant, but also to rule out any disorders or health issues.



  1. Hi ,
    I had sex with my GF last month. After 8-10 days she got her period as normal. Now its her second period time and its almost 3 days late. Is there any chance to get pregnant ?

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