Common menstrual irregularities and what causes them

Defining the normal menstrual cycle

Understanding what normal looks like helps identify what is abnormal. The menstrual cycle has significant natural variation between women and within the same woman across her reproductive life.

Cycle length

A normal menstrual cycle ranges from 21 to 35 days, measured from the first day of one period to the first day of the next. Most cycles are 28 days but variation within the 21 to 35 day range is normal. Cycles shorter than 21 days or longer than 35 days warrant investigation. Cycles are typically longer in adolescence and early adulthood and become more regular through the twenties and thirties before becoming variable again approaching menopause.

Period length and flow

Most periods last 3 to 7 days. Total blood loss typically ranges from 30 to 80 ml across a period, though women generally do not measure this. Flow varies through the period, typically heavier in the first 1 to 2 days and lighter towards the end. Some clotting is normal. The amount and pattern that is normal varies significantly between women.

Cycle to cycle variation

Some variation between cycles is normal. Cycles within the same woman can vary by 7 to 9 days and still be considered regular. Significant variation may indicate hormonal issues. Stress, illness, travel and lifestyle changes can all temporarily affect cycle timing. Persistent significant variation warrants assessment.

Life stage variations

Cycles in the first 2 to 3 years after periods start (menarche) are often irregular as the hormonal system matures. Cycles in the 5 to 10 years before menopause (perimenopause) become irregular again as ovarian function changes. Pregnancy, breastfeeding and use of hormonal contraception all affect cycles. These life stage variations are physiological.

Types of menstrual irregularity

Several specific patterns of menstrual irregularity have medical names. Understanding the categories helps describe symptoms to your GP and informs the appropriate investigation.

Heavy menstrual bleeding

Heavy bleeding (menorrhagia) is bleeding heavy enough to interfere with daily activities. Signs include needing to change pad or tampon every 1 to 2 hours, passing large clots, bleeding through clothes or bedding and feeling tired or breathless during periods. Causes include uterine fibroids, polyps, hormonal imbalance, bleeding disorders and certain medications. Heavy periods can cause iron deficiency anaemia and warrant assessment.

Absent or missed periods

Amenorrhoea is the absence of periods. Primary amenorrhoea is when periods have not started by age 15. Secondary amenorrhoea is when periods stop for 3 or more months in someone who previously had regular cycles. Causes include pregnancy, breastfeeding, hormonal contraception, low body weight, excessive exercise, polycystic ovary syndrome, thyroid problems and premature ovarian insufficiency. Missed periods warrant investigation.

Irregular cycles

Cycles that vary significantly in length, with periods coming unpredictably. Some irregularity in adolescence and perimenopause is normal. Persistent irregularity in the reproductive years may indicate polycystic ovary syndrome, thyroid issues, hormonal imbalance, stress related cycle disruption or other conditions. Tracking cycles for several months helps identify patterns and gives useful information to share with a GP.

Bleeding between periods

Bleeding or spotting between periods (intermenstrual bleeding) warrants investigation. Common causes include hormonal contraception (particularly in the first months of use), polyps, fibroids, infection, cervical changes and rarely cancer. Bleeding after sex, after menopause or with significant pain particularly warrants prompt assessment. Most causes are benign but assessment is important.

What drives menstrual irregularities

Menstrual irregularities have many possible causes. Some are temporary and resolve without treatment. Others reflect underlying conditions that benefit from medical management.

Hormonal imbalance

The menstrual cycle is regulated by complex hormonal interactions. Imbalances in oestrogen, progesterone, FSH, LH or other hormones can disrupt the cycle. Common hormonal causes of irregularities include polycystic ovary syndrome (PCOS), thyroid disorders, hyperprolactinaemia and perimenopause. Blood tests can identify many hormonal causes.

Structural conditions

Conditions affecting the structure of the uterus and surrounding organs can cause irregular bleeding. Uterine fibroids (benign growths in the uterus muscle), polyps (growths in the uterine lining), endometriosis (uterine tissue growing outside the uterus) and adenomyosis (uterine tissue growing into the muscle wall) all can produce irregularities. Ultrasound and other imaging can identify these conditions.

Lifestyle factors

Stress, significant weight changes, excessive exercise, sleep disruption and travel can all affect cycles. Athletes with high training loads and low body fat often experience cycle disruption. Eating disorders cause severe cycle disruption. The hypothalamus is sensitive to physical and psychological stress and disrupts cycle hormones when stressed. Addressing the underlying lifestyle factor often resolves the cycle issue.

Medications and contraception

Hormonal contraception affects cycles in various ways. The combined pill typically produces regular withdrawal bleeds. Progesterone only methods can produce irregular bleeding or absent periods. The IUD can cause heavier bleeding initially. Other medications including thyroid drugs, antidepressants and certain blood thinners can affect cycles. Discuss any concerning changes with your GP.

Signs that warrant assessment

Many menstrual irregularities benefit from medical assessment. Specific patterns particularly warrant prompt attention.

Heavy bleeding

See your GP if bleeding is heavy enough to interfere with daily activities, soaks through pads or tampons within an hour, contains large clots regularly, lasts more than 7 days or makes you feel tired or short of breath. Heavy bleeding has effective treatments and should not be tolerated. Iron levels should be checked because heavy periods commonly cause iron deficiency anaemia.

Missed periods

See your GP if periods stop for 3 or more months in someone with previously regular cycles (and pregnancy is not the cause). Earlier consultation may be appropriate if other symptoms are present. Missed periods may indicate hormonal imbalance, weight related cycle disruption, polycystic ovary syndrome, thyroid issues or other conditions that benefit from identification and management.

Significant cycle changes

See your GP if the cycle pattern has changed significantly from your usual pattern. Cycles that suddenly become much shorter, longer, more painful or have different bleeding patterns warrant assessment. Changes in flow including significant heavier bleeding or new clotting should be discussed. Tracking cycles for several months helps document the changes.

Postmenopausal bleeding

Any bleeding after menopause (defined as 12 months without periods) should be assessed promptly. Postmenopausal bleeding has many possible causes including HRT, vaginal atrophy and polyps. Some causes including endometrial cancer need prompt diagnosis. Postmenopausal bleeding does not usually indicate cancer but always warrants investigation.

More on female health

For the cycle fundamentals our The Menstrual Cycle: A Complete Guide covers normal physiology. The Four Hormonal Phases of the Menstrual Cycle covers the hormonal pattern. And Iron Deficiency in Women covers a common consequence of heavy periods.