Why do menstrual cycles vary?

Menstrual cycles are dynamic. They can vary for many reasons and in many ways. These variations are usually normal and healthy. In some cases though, variations point to something more serious, like a medical condition that needs your attention.

Having a menstrual cycle is like having an extra vital sign — like your pulse or body temperature. Your cycle can tell you when you’re in your usual rhythm, when something is a bit off or when you may have a medical condition that could need treatment.

Clinically speaking, cycles are described in two ways: regular and irregular. This refers to how many days a cycle lasts and how a cycle’s length changes over time. There are also regular and irregular ranges for menstrual bleeding, and regular and irregular ranges for pain.

Reproductive hormones

Menstrual cycles are the rhythmic ups and downs of your reproductive hormones, and the physical changes those ups and downs cause. They trigger the growth of follicles in the ovaries, the release of an egg (ovulation) and the growth and shedding of the uterine lining (the period). The reproductive hormones include estrogen, progesterone, follicle stimulating hormone, testosterone and others.

These hormones play a role well beyond reproduction. They affect everything from your sleep, mental health and weight to your bone density and heart health (1–3).

In a way, hormones in the menstrual cycle act a bit like they are in a relay race. As the cycle moves forward, one hormone often triggers the next, which then triggers the next, moving the cycle through its different phases (menstruation, follicular phase, ovulation, luteal phase).

Too much or too little of certain hormones can stop the cycle from working as it should. If one hormone doesn’t “pass the baton,” things can slow down or stop altogether. It’s a delicate and important balance (4).

Anything that affects the balance of your reproductive hormones can affect your cycle’s length, symptoms and the length and heaviness of your period.

Regular variations

Most cycles are between 24–38 days long in adults, but cycles will often have variations. For example, if you are stressed during the first half of your cycle, your ovulation may happen later than usual. Progesterone (the hormone dominant after ovulation) may then take longer to peak, causing certain symptoms (such as sore breasts) to happen later. Your period may then be a couple of days late (5).

These types of variations are extremely common and happen in most peoples’ cycles (5, 6). They can be caused by everything from diet and sleep changes to jet lag and smoking (3, 7).

Irregular variations

Bigger variations also occur. These are cycles that fall outside of “regular” ranges, or vary in length by more than 7–9 days cycle-to-cycle. Irregular cycles are often temporary, lasting only one or two cycles. Temporary irregularities can happen for reasons such as a miscarriage, high stress, or changing brands of hormonal contraception. If you don’t ovulate at all in one cycle, for example, that cycle may be longer with a slightly lighter period (your hormones and symptoms will be affected as well) (5). Temporary irregularities in the menstrual cycle are usually nothing to worry about (5, 6, 8).

Irregular cycles can also be longer lasting/continuous. Long-term irregularities can happen in response to things such as working night shifts and high-intensity exercise or due to medical conditions such as polycystic ovary syndrome (4, 5, 8, 9). Many people have undiagnosed medical conditions which affect their cycle (10). Periods that are very heavy, light or painful may also signal an issue — endometriosis, for example, is a common (and under-diagnosed) cause of painful menstruation.

Cycles that consistently fall outside of “normal” ranges are a reason to check-in with a healthcare provider — they may have an impact on a person’s long-term health, including their bone density, heart health, sleep, fertility and more. Unmanaged medical conditions may have their own additional complications (1, 2, 10).

Irregular cycles are normal and expected when your periods first begin in life (called menarche) and when they come to an end (called perimenopause). In between these times, cycles that are consistently irregular should be addressed with a healthcare provider. Spotting (bleeding that occurs outside of menstruation) should always be addressed with a healthcare provider (some people have consistent spotting at the time of ovulation, which is probably not a concern) (11).

Know your cycle

Learning what your average cycle looks like will help you identify when variations — of any type — occur. You’ll be able to spot a potential pregnancy sooner, or notice when something has caused your cycle to change.

Tracking other categories, like sleep and stress, will help you identify what things tend to impact your cycle. If your cycle becomes (or is always) irregular, the information you gather with Clue may provide you and a healthcare professional with information that could help in the diagnosis and management of a medical condition. It may help you come up with an individualized treatment plan, or identify a change like menopause or pregnancy.

What you can do about it

Regulating an irregular menstrual cycle begins with figuring out the reason behind your symptoms or irregularity. Then you can address the cause directly.

Your healthcare professional will likely ask you questions about your daily activities and health history, and may give you an abdominal ultrasound and/or hormonal profile test. For a hormonal test, you’ll be asked what day of your cycle you’re on (bring your Clue data with you) and have some blood drawn. You may then learn if your hormones are in balance, and even whether or not you are ovulating.

Treatment will depend on the cause and type of your irregularity. It may include anything from hormonal medication and behavioral or dietary changes, to supplements that promote ovulation (12, 13).

Start tracking variations in your cycle today.

Common reasons for cycle irregularities:

Life stages and pregnancy

  • After menarche (the years after menstruation begins)
  • Perimenopause (as cycles come to an end)
  • Pregnancy
  • The postpartum period (after a pregnancy)
  • Miscarriage (known or unknown)
  • Abortion


  • Changing or stopping hormonal birth control
  • IUDs
  • Emergency contraception

Sleep/Wake Cycles

  • Shift work or working night shifts
  • Sleep disorders
  • Jet lag/long distance travel

Physical/Emotional Changes

  • Stress
  • Big emotional changes, such as grief
  • Quick weight loss
  • Not getting enough calories
  • Intensive exercise
  • Certain medications

Medical conditions (short list)

  • Polycystic ovary syndrome
  • Endometriosis
  • Pelvic inflammatory disease (caused by an untreated sexually transmitted infection)
  • Uterine polyps
  • Uterine fibroids
  • Thyroid disorders
  • Unmanaged diabetes
  • Primary ovarian insufficiency
  • Bleeding disorders

Causes for irregular or intense pain:

  • Endometriosis
  • Ovarian cysts
  • Ectopic pregnancy
  • Miscarriage
  • PMS combined with smoking, heavy flow, or inflammation
  • Infection, such as pelvic inflammatory disease (caused by an sexually transmitted infection)


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