Perimenopause is common — but not the only explanation. Here's what else to consider.

1. Thyroid disorders
   The thyroid is one of the most under-diagnosed causes of menstrual irregularity. Hypothyroidism
   (underactive thyroid) causes longer, heavier periods because low thyroid hormone slows the
   hormonal cascade that governs the menstrual cycle. Hyperthyroidism (overactive thyroid) tends to
   cause shorter, lighter periods — or missed periods altogether. Thyroid disorders are also common
   in the 40–55 age group, making them easy to confuse with perimenopause. A simple TSH blood
   test can rule this out.
   
   

2. Polycystic ovary syndrome (PCOS)
   PCOS is characterised by excess androgens (male hormones) that suppress regular ovulation.
   Without reliable ovulation, the uterine lining builds up unevenly and sheds in irregular bursts —
   causing cycles that stretch to 60, 90, or even 120 days, or disappear for months entirely. PCOS
   affects 6–8% of reproductive-age women and is a leading cause of long cycles in women under 40.
   Importantly, women with PCOS are 65% more likely to develop uterine fibroids — meaning both
   conditions can coexist and compound cycle disruption.
   
   

3. Uterine fibroids
   Fibroids — non-cancerous muscle growths in or on the uterine wall — affect up to 80% of women
   by age 50 and are most common in the 30s and 40s. They disrupt the menstrual cycle by altering
   how the uterine lining builds and sheds, and by pressing on surrounding tissue. Symptoms include
   heavier bleeding, prolonged periods, pelvic pressure, and increased urinary frequency. Fibroids are
   oestrogen-dependent and often worsen during perimenopause — then typically shrink by 50% or
   more after menopause when oestrogen drops.
   
   

4. Uterine polyps
   Endometrial polyps are small, usually non-cancerous overgrowths of the uterine lining. They cause
   unpredictable and prolonged bleeding, spotting between periods, and irregular cycles. They are
   more common in women aged 40–50 and are typically diagnosed by transvaginal ultrasound or
   hysteroscopy. Most are benign, but any postmenopausal bleeding from polyps requires
   investigation.
   
   

5. Chronic stress
   High or prolonged stress triggers elevated cortisol, which directly suppresses GnRH
   (gonadotropin-releasing hormone) — the master signal that initiates the ovulatory cascade.
   Without sufficient GnRH signalling, ovulation is delayed or skipped, stretching the cycle. This
   mechanism explains why significant life stress — bereavement, divorce, job loss, major illness —
   often precedes missed or delayed periods even in women with previously regular cycles.
   
   

6. Significant weight changes
   Both significant weight gain and weight loss can disrupt the menstrual cycle. Fat tissue is a site of
   peripheral oestrogen production — a marked increase in body fat raises oestrogen levels in a way
   that can suppress ovulation. Conversely, very low body weight or rapid weight loss reduces oestrogen, also disrupting the cycle. A BMI below 18.5 or above 30, or a rapid change of more than
   10% body weight, is associated with menstrual irregularity.
   
   7. Medications and IUDs
   Several common medications alter cycle length as a side or direct effect. Copper IUDs are a well-
   known cause of heavier, longer, and more irregular bleeding. Blood thinners (anticoagulants),
   antipsychotics, some antiepileptics, and high-dose corticosteroids can all affect cycle timing and
   flow. If a cycle change coincided with starting a new medication, this is worth discussing with your
   prescriber.

Read more: Irregular periods: causes and treatments — Cleveland Clinic