How long is an average menstrual cycle? While averages give us a framework, they overlook occasional variations. The number of days of your cycle can sometimes fluctuate. What causes an irregular period? Stress, diet, exercise, and other lifestyle factors can affect the length of your cycle and don’t immediately signal dysfunction. Here we’ll uncover the factors contributing to variations.

What is the “Average” Length of a Menstrual Cycle? 

The length of a menstrual “cycle” is measured from day one of your bleed until the day before your next bleed begins. It is common for menstruators (who are not on hormonal birth control pills) to experience a 24-to-38-day cycle[i].

There are several instances when variations are common, including:

  • Your first period, or menarche. It can take several years for our average cycle length to emerge. 
  • If you’re on hormonal contraception. You experience a 26-to-28 day “cycle”, but this is, in fact, not shedding the lining of the uterus. Instead of menstrual bleeding, it is withdrawal bleeding, caused by pausing the hormonal pills for 7 days of placebo[ii].  
  • After birth or while you’re breastfeeding.  
  • During perimenopause, as your body prepares for menopause. 

What if My Cycle is Short/Long/Non-Existent?

Polymenorrhea describes a cycle of less than 21 days. Oligomenorrhea is a cycle length of 38 to 90 days. Amenorrhea is the loss of a period all together. An occasional variation of >5 days doesn’t necessarily signal dysregulation or an underlying condition[iii]. If you experience an irregular period for several months, check in with your healthcare provider.

What Causes an Irregular Period?

Irregular periods are common with stress, lack of nutrition, and overexercising[iv]. Extreme exercising and dieting can cause estrogen levels to drop. The sex hormones, estrogen and progesterone, maintain specific ratios for hormonal balance and cycle regulation.

Too little estrogen can cause shorter cycle lengths. Inadequate amounts of progesterone can mean longer cycles. Both can lead to adverse symptoms like heavier flows, more intense cramps, anxiety, loss of sex drive, hair thinning, hot flashes, and infertility issues[v]. These hormones are intended to work in a symbiotic way.

Diet & Exercise

An increase in oligomenorrhoea and amenorrhea in athletes and individuals with eating disorders has been studied. Dancers account for 11-44% of the population that experience oligomenorrhoea and 6-60% of athletes report experiencing it at some point in their life.

Oligomenorrhoea is characterized as a repeated menstrual cycle greater than 35 days or only 4-9 cycles in total in a year. Oligomenorrhoea is found in 13.5% of the menstruating population. In menstruators of reproductive age, polycystic ovarian syndrome (PCOS) accounts for 4-10% of that[vi].

Stress

Similarly, individuals in high stress jobs have reported experiencing shorter cycles. When the body produces too much cortisol, it can disrupt the hypothalamus of the brain and interrupt the menstrual cycle. Sedentary lifestyles have also been linked to shorter menstrual cycles. Finding balance is therefore pivotal to hormone balancing. 

Other Contributing Factors

Other factors contributing to cycle length variations include:

  • poor nutrition 
  • irregular sleep patterns (jet lag, shift-workers) 
  • emergency contraception pills 
  • cigarettes 
  • alcohol consumption

The best way to respond to cycle variations is to look at your current lifestyle. Have you been highly stressed? Are you eating properly? Are you exercising a moderate amount leaving you feeling energized and not drained? Have you experienced significant weight loss?

Periods are highly affected by our habits and routines. To set yourself up for period wellness, make note of the date your menstrual cycle begins and ends, and mark any changes over the course of several months. If you experience one cycle of 45 days, for example, keep an eye on your next cycles and work to balance the lifestyle factors contributing to your hormone levels.

Polycystic Ovarian Syndrome (PCOS); What Is It?

Historically, health care providers have dismissed menstruators for cycle related symptoms. As a result, disorders such as polycystic ovary syndrome (PCOS) have gone grossly underdiagnosed[vii].

Polycystic ovary syndrome is a common endocrine disorder of menstruators of reproductive age. PCOS has 4 different types, and other than an irregular period, its often characterized by symptoms like hair loss, acne, hirsutism, uncontrollable weight gain, and infertility[viii]. Do not hesitate to reach out to a trusted obstetrician or gynecologist for proper diagnosis and treatment.

Living a Balanced Life

Living a balanced life means exercising moderately, nourishing your body well, monitoring stress levels, and seeking help in addressing underlying medical conditions if necessary.

Understanding the ins and outs of your cycle – and talking about it openly – helps you to better understand your experience as a menstruator and contributes to the de-stigmatization of periods.


[i] What’s “normal”?: Menstrual cycle length and variation (helloclue.com) 
ii] Ibid. 
[iii] Length and variation in the menstrual cycle—a cross‐sectional study from a Danish county – MÜNSTER – 1992 – BJOG: An International Journal of Obstetrics & Gynaecology – Wiley Online Library
[iv] What causes menstrual irregularities? | NICHD – Eunice Kennedy Shriver National Institute of Child Health and Human Development (nih.gov)
[v] Oligomenorrhea – StatPearls – NCBI Bookshelf (nih.gov)
[vi] Vitti, A., 2020. In the FLO. New York: HarperCollins Publishers, pp.183-185.
na. 8 Steps to Reverse Your PCOS: A Proven Program to Reset Your Hormones, Repair Your Metabolism and Restore Your Fertility. Austin, Tx, Greenleaf Book Group Press, 2016. 
[viii] Ibid.