The menstrual cycle lasts from the first day of a person’s period to the day before the next period arrives. To an extent, it is normal for a person’s cycle length to vary from month to month.

The Office on Women’s Health (OWH) suggest that the average menstrual cycle lasts 28 days. However, a 2019 study found that the average cycle length is a little more than 29 days.

In this article, we will look at what a normal period is, causes of a long or short cycle, and how someone can track the regularity of their cycle.

a woman on her phone using an application to track how many days between periodsShare on Pinterest
A person can try tracking their periods using a mobile application.

The lining of the uterus thickens each month to prepare for pregnancy. If a fertilized egg does not implant in the uterus, the uterine lining sheds, causing a period. The period is the first part of a person’s cycle and marks the end of the previous cycle.

It is usual for the number of days between periods to vary. A normal range could fall anywhere between 24–38 days. Doctors may call a cycle that falls outside this time frame irregular. Doctors may also call a period irregular if it varies by more than 20 days in length from month to month.

However, cycle variation does not necessarily signal a problem.

There are several potential reasons why a person might experience an unusual amount of days between periods.

A 2019 study that analyzed more than 600,000 menstrual cycles found that the most important factor determining cycle length is the time it takes for a person to ovulate. Ovulation occurs when the ovaries release an egg. The average person ovulates about 16 days after their last period.

If someone ovulates early, their cycle may be shorter. If they ovulate later, the cycle may be longer. How often a person ovulates can change depending on several factors, including:

  • Age: Adolescents may not ovulate regularly to begin with, so the length of their cycles can fluctuate. Similarly, people entering menopause will start to ovulate less often, causing changes to period and cycle length.
  • Stress: A 2013 study of shift workers found that menstrual cycle disorder was more common in people who had stressful jobs and irregular working hours.
  • Energy levels: Intense exercise and eating disorders, such as anorexia nervosa or bulimia, can mean a person stops ovulating, causing irregular periods.

According to an older article in the Journal of Clinical Endocrinology & Metabolism, both long and short cycles can indicate that a person did not ovulate during that cycle.

Causes of a short cycle

People who ovulate earlier in their cycle often have fewer days between periods. Some specific causes for a short cycle include:

  • Endometriosis: A meta-analysis found that cycles shorter than 27 days correlate with a higher incidence of endometriosis. Symptoms of endometriosis include severe menstrual cramps, long periods, and heavy bleeding.
  • Luteal phase defect (LPD): The luteal phase is the second half of the menstrual cycle. According to Resolve, some doctors believe that an LPD could cause a short luteal phase, making it difficult for a fertilized egg to implant in the womb lining. However, there is some debate among experts about whether LPD exists.
  • High body mass index (BMI): In one study, women with a high BMI were more likely to have a short luteal phase, and 34% less likely to get a positive result when researchers tested if they had ovulated.
  • Birth control: Sometimes, a person using hormonal birth control will experience “breakthrough bleeding.” This type of bleeding is not a true period, but rather a side effect of the birth control.

Causes of a long cycle

The later into their cycle a person ovulates, the later their period will be. A long cycle may mean that a person ovulated very late, or did not ovulate at all.

According to the OWH, specific causes for a long menstrual cycle include:

  • Polycystic ovary syndrome (PCOS): Sometimes, late ovulation is a sign of PCOS. Other symptoms of PCOS include irregular periods, unusual or excessive body hair growth, unexplained weight gain, and insulin resistance.
  • Hyperthyroidism: When the thyroid gland produces too much thyroid hormone, it can cause late or missed periods. The OWH list other hyperthyroidism symptoms as feelings of nervousness, unexplained weight loss, a rapid heartbeat, and trouble sleeping.

A person with these symptoms should speak to their doctor. If a person has not had their period, they should also consider whether they might be pregnant.

Period tracking can help a person predict their next period. It can also help a person determine whether their cycle is regular or irregular.

There are a few ways a person can track their period, including mobile applications or in a diary or calendar.

Some people also use period tracking to monitor when they might ovulate. However, according to an article in npj Digital Medicine, period tracking alone is not reliable, since the day of ovulation can change month to month. Additionally, a typical cycle length does not necessarily mean a person ovulated.

According to an article in Bioengineering and Translational Medicine, if a person wants to find out when they ovulate, they should also monitor:

  • Cervical fluid: The cervical fluid that comes out of the vagina changes during a person’s cycle. When a person is most fertile, the fluid tends to be more abundant and watery and may resemble egg whites.
  • Basal body temperature: Recording body temperature each morning can help confirm ovulation. When the temperature rises for several days and remains elevated, this may mean a person has ovulated. This method is most effective if a person takes their temperature at the same time each day before moving around.
  • Ovulation tests: Ovulation tests measure for surges of luteinizing hormone. A positive test means a person may be about to ovulate. However, some disorders can cause hormone surges even when a person does not ovulate.

Because an irregular period sometimes means a person is not ovulating, very long or short cycles could be a sign of a hormonal imbalance that may make it difficult to conceive.

However, it is not necessarily the case that someone with irregular periods will not get pregnant, or that someone with regular periods will. The OWH lists several causes of infertility in males and females.

People who have irregular periods who want to get pregnant may want to see a doctor before trying to conceive.

People should also inform a doctor if they experience sudden changes in their cycle or have symptoms of an underlying condition, such as severe cramps or heavy periods.

The average number of days between periods is 28–29. However, it is normal for peoples’ cycles to vary from month to month. A significant amount of variation could be a sign of an underlying condition or hormonal imbalance.

If a person has concerns about the number of days between periods or their fertility, they should contact a doctor.