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Katrin Friedman

Fertility

Why do menstrual cycles vary?

by Anna Druet, Former Science and Education Manager
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Menstrual cycles are dynamic. They can vary for many reasons and in many ways. The length of your cycle, the heaviness of your period, and the symptoms you experience can all fluctuate. These variations are usually normal and healthy. In some cases though, they can 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 a cycle's length, and how much cycle length varies, cycle-to-cycle. There are also regular and irregular ranges for menstrual bleeding, and regular and irregular ranges for pain.

No one is perfectly "regular"

A period that comes every cycle at exactly the same time is not the norm. Of course cycles are irregular when they’re first starting, as they’re coming to an end, and around the times of pregnancy and changes to contraceptive methods. But even outside of those times, slight variations in timing and symptoms are common. If you are stressed out during the first half of your cycle, for example, your ovulation may happen a couple of days later than usual. Progesterone (the hormone that’s dominant after ovulation) will then peak later in your cycle. This will cause certain symptoms (such as sore breasts) to happen later as well. Your period will also then be a couple of days late (1).

These types of variations are extremely common and happen in most peoples’ cycles (1,2). They can be caused by changes in your environment, behavior, or health, including things like diet and exercise, sleep changes and jetlag and smoking (3,4). Slight changes in your cycle length, period length, and period volume are also normal over time (1)

Why does variation happen?

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

Menstrual cycles are caused by 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, luteinizing hormone, testosterone and others.

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. Having more or less of certain hormones will create changes in the pace and timing of the cycle. If one hormone doesn’t “pass the baton,” things can slow down or stop altogether. It’s a delicate and important balance (5).

Regular ranges: what’s normal?

A cycle is considered clinically regular when the majority of someone’s cycles and periods are within these ranges:

  • Adolescents: Cycles within the range of 21–45 days (6)
  • Adults: Cycles within the range of 24–38 days (7)
  • Adults: Cycles that vary in length by up to 7–9 days (for example, a cycle that is 27 days long one month, 29 the next) (8)
  • Periods that last 8 days or fewer (8)

Regular mid-cycle spotting may be caused by ovulation (1). Any unpredictable spotting or continual changes to period length should be addressed with a healthcare provider (8). If the majority of your cycles fall outside these ranges, read more here.

Noticing bigger changes in your cycle

Variations that are more significant also occur. They are often temporary, lasting only one or two cycles. These can happen for reasons such as an undetected miscarriage, high stress, or not getting enough calories. If you don’t ovulate at all in one cycle, for example, that cycle may be longer with a slightly lighter period (1). The cycle and period following that one may also be a bit different. Temporary irregularities in the menstrual cycle are usually nothing to worry about (1,2,9).

But irregular cycles can also be longer lasting. These are cycles that continually fall outside of “regular” ranges (or the majority of cycles in the past 6 months). 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 (3,8,9,10). Many people have undiagnosed medical conditions which affect their cycle (10). Periods that are very heavy, or painful may also signal an issue—endometriosis, for example, is a common (and underdiagnosed) cause of painful menstruation.

Reproductive hormones play a role well beyond reproduction. They affect everything from your sleep, mental health and weight to your bone density and heart health (11-12). Cycles that consistently fall outside of "normal" ranges should be addressed with a healthcare provider.

Read more about irregular cycles and their causes.

Know your cycle

Learning what your average cycle looks like will help you identify when variations - of any type - occur. Tracking other things 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 your healthcare provider 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.

Common reasons a "regular" cycle can vary:

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 (13)

Contraceptives

  • Changing or stopping hormonal birth control
  • IUDs (hormonal or copper)
  • Emergency contraception (14-16)

Sleep/Wake Cycles

  • Shift work or working night shifts
  • Sleep changes
  • Jet lag/long distance travel (3)

Physical/Emotional Changes

  • Stress
  • Substance use
  • Diet changes
  • Not getting enough calories
  • Intensive exercise
  • Substance use
  • Big emotional changes, such as grief
  • Quick changes in weight
  • Certain medications (17-20)

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