If you’re not pregnant, your period may be late due to a great deal of stress or weight changes. Certain conditions, including polycystic ovary syndrome (PCOS) and diabetes, can also affect your cycle.

Some research suggests that the prevalence of irregular menstrual cycles ranges from 5% to 35.6%, depending on age, occupation, and location.

There are also two times when it’s typical for your period to be irregular: when it first begins in adolescence and when it comes to an end during the perimenopausal transition to menopause.

Most people who haven’t reached menopause have a period approximately every 28 days. However, a typical menstrual cycle can range from every 21 to 35 days.

If your period doesn’t fall within these ranges, it could be because of one of the following reasons.

Chronic stress can throw off your hormones, change your daily routine, and even affect the part of your brain responsible for regulating your period: your hypothalamus.

Over time, stress can lead to illness or sudden weight gain or loss, all of which can impact your cycle.

Rapid or excessive weight loss — such as with disordered eating habits or eating disorders — can cause irregular periods and may even stop your cycle altogether.

Regularly engaging in extreme exercises, such as marathons, may also cause irregularities.

In the same way that living with a lower body weight can cause hormonal changes, living with a higher body weight can also cause irregularities.

Obesity can cause the body to produce an overabundance of estrogen, which is a key reproductive hormone. Too much estrogen can cause irregularities in your cycle and may even stop your periods altogether.

If a healthcare professional has determined that obesity is a factor in your late or missed periods, they may advise you to lose weight through lifestyle changes, such as focusing on nutrient-dense foods and exercising.

Starting or stopping certain birth control methods can cause visible changes in your menstrual cycle. This includes the following:

  • combination pill
  • progestin-only mini-pill
  • implant
  • IUD
  • skin patch
  • shot
  • vaginal ring

With hormonal birth control, many people experience period-like bleeding — also known as breakthrough or withdrawal bleeding — in place of menstruation.

The copper IUD doesn’t prevent you from ovulating or subsequently menstruating. However, it can cause you to experience longer or shorter menstrual cycles and other irregularities.

An overactive or underactive thyroid gland could also cause late or missed periods. The thyroid regulates your body’s metabolism, which can affect your hormone levels.

Thyroid conditions can usually be treated with medication.

PCOS causes the body to produce more androgen. Cysts form on the ovaries as a result of this hormone imbalance. This can make ovulation irregular or stop it altogether.

Other hormones, such as insulin, may also become unbalanced. This is due to insulin resistance, which is often associated with PCOS.

Treatment for PCOS focuses on relieving symptoms. A healthcare professional may prescribe birth control or another medication to help regulate your cycle.

Most people begin menopause between the ages of 45-55. Those who develop symptoms around age 40 or earlier may be experiencing POI or early natural menopause.

About 1% of people before the age of 40 experience POI. While this condition can arise from the surgical removal of the ovaries, other causes include genetic disorders and autoimmune conditions.

If you’re experiencing missed periods and other POI symptoms, make an appointment with a healthcare professional to talk about testing and treatment.

Chronic diseases, such as diabetes and celiac disease, can also affect your menstrual cycle.

Changes in blood sugar are linked to hormonal changes, so even though it’s rare, unmanaged diabetes could cause your period to be irregular.

Celiac disease causes inflammation that can lead to damage in your small intestine, which may prevent your body from absorbing key nutrients. This can cause irregular or missed periods.

Other chronic conditions that may lead to cycle irregularities include:

If your periods seem irregular, or you’ve missed a period but know you’re not pregnant, it’s a good idea to make an appointment with a healthcare professional.

If you can, keep a record of menstrual changes and other symptoms you’ve noticed. Your healthcare professional can use this information to help narrow down the underlying cause.

Seek urgent medical care if you experience any of the following:

  • heavy bleeding
  • fever
  • severe pain
  • nausea and vomiting
  • bleeding that lasts longer than 7 days
  • bleeding after menopause

Just as every person is different, every menstrual cycle is different. While 28 days is the generalized cycle length, cycles can range from 28 to 35 days.

Occasional cycle irregularities can happen for a variety of reasons, from chronic stress to weight fluctuations to stopping or starting birth control.