Studies reveal: actual length of the menstrual cycle

What is a normal length of a menstrual cycle and how much can it fluctuate without giving cause for concern?

For some lucky women and other people with a vulva the length of their menstrual (or hormonal) cycle is exactly the same from month to month, year to year. For others, it can vary a lot. Many people find that the length of their cycle changes with age.

Length of menstrual cycle

The average length of a menstrual cycle is between 28 and 29 days.* The length is calculated from the beginning of your period to the last day before your next period starts. The first proper day of menstruation is always day 1 of the cycle. If you experience light spotting for a day or two, this is not yet considered the start of menstruation; these days are still part of the previous cycle.

The length of your cycle may differ significantly from this average, without any cause for concern. Different sources in different countries define the normal cycle in slightly different ways – even in Finland, some refer to a 24-38 day cycle, others to 23-35 days. Menstruation itself normally lasts 2-7 days. 

How much can the length of the cycle vary?

For most people, the length of the menstrual cycle varies from one month to the next by 1-3 days. Even variation of a week is within the bounds of normal. Many people have more irregular periods and longer periods in their teens. About three years after the start of menstruation, the length of your cycle usually stabilises. 

At the other end of the reproductive age, menstrual cycles often return to irregular. This usually happens between the ages of 40 and 50, as you approach menopause. Typically, the cycle first shortens and then lengthens. It is also common for the length of menstrual cycles to fluctuate widely during perimenopause. Sometimes periods come once a month, sometimes every three months. You may find that irregularity is accompanied by heavy bleeding.

Read more about heavy periods.

Certain changes in your health and body, such as significant and rapid weight fluctuations, stress, hormonal imbalances or illnesses, can make your period irregular.

If you are worried about the length or regularity of your period, talk to your doctor. Your periods may also interfere with normal life, especially if they are irregular, in which case it’s a good idea to talk to your gynaecologist. There are often good treatments for menstrual problems.

*The average length of a menstrual cycle is traditionally reported as 28 days, but in a large study in 2019 the median length of a menstrual cycle was actually 29 days (Bull, J.R., Rowland, S.P., Scherwitzl, E.B. et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digit. Med. 2, 83 (2019).)

Sources: NHS; OWH; Terveyskirjasto; Naistalo