Common Causes of Menstrual Cycle Irregularities

Pregnancy and Breastfeeding

Missed or delayed periods are often one of the earliest signs of pregnancy. Women experiencing sudden changes in their menstrual cycle after unprotected sexual activity should consider taking a pregnancy test to confirm.

During breastfeeding, menstrual cycles may also be delayed due to prolactin, the hormone responsible for milk production. Elevated prolactin levels suppress reproductive hormones such as estrogen and progesterone, resulting in lighter periods or temporary amenorrhea (absence of menstruation).

Eating Disorders, Excessive Exercise, and Extreme Weight Loss

Rapid weight loss, extreme dieting, and eating disorders such as anorexia nervosa or bulimia can disrupt the menstrual cycle.

• Women with a body mass index (BMI) below 18.5 are particularly vulnerable
• Insufficient calorie intake interferes with luteinizing hormone (LH) and follicle-stimulating hormone (FSH), both crucial for ovulation

Additionally, intense physical activity or over-exercising can further suppress reproductive hormone production, leading to missed periods, irregular cycles, or amenorrhea. Maintaining a balanced diet and healthy body weight is vital for regular menstrual cycles.

Polycystic Ovarian Syndrome (PCOS)

Polycystic Ovarian Syndrome (PCOS) is a common hormonal disorder characterised by the presence of multiple small cysts in the ovaries. Women with PCOS often experience:

• Irregular or absent ovulation
• Elevated levels of testosterone, causing hormonal imbalance
• Symptoms such as acne, excess hair growth (hirsutism), and weight gain

PCOS is one of the most common underlying causes of menstrual cycle irregularities and may affect fertility if left untreated. According to the Office on Women’s Health, about 1 in 10 women with irregular menstrual cycles has PCOS. Medical management, lifestyle modifications, and sometimes medication are essential for regulating cycles and improving reproductive outcomes.

Premature Ovarian Failure (POF)

Premature ovarian failure, also known as ovarian insufficiency, occurs when the ovaries lose normal function before age 40. Women with POF may experience:

• Irregular or skipped periods over several years
• Difficulty conceiving naturally
• Early onset of menopause-like symptoms such as hot flashes and low estrogen levels

This condition can significantly impact fertility, and early medical evaluation is recommended for diagnosis and management.

Pelvic Inflammatory Disease (PID)

Pelvic Inflammatory Disease (PID) is an infection of the female reproductive organs, commonly caused by sexually transmitted bacteria. If untreated, PID can damage the fallopian tubes and uterus, leading to chronic pain and infertility.

Common symptoms of PID include:

• Bleeding between periods or after sexual intercourse
• Pelvic or lower abdominal pain
• Unusual vaginal discharge

Early diagnosis and antibiotic treatment are critical to prevent long-term complications and preserve reproductive health.

Uterine Fibroids

Fibroids are noncancerous tumours that develop in the wall of the uterus. They vary in size, from as small as an apple seed to as large as a grapefruit. Fibroids can disrupt menstrual cycles and cause:

• Heavy menstrual bleeding, potentially leading to anemia
• Pain during intercourse
• Pelvic pressure or discomfort

Fibroids are common in women of reproductive age and are one of several structural causes of menstrual cycle irregularities, often requiring medical or surgical intervention depending on severity and symptoms.

Thyroid Disorders

Your thyroid gland produces hormones that regulate metabolism and play a direct role in your menstrual cycle. Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) are among the most commonly cited causes of menstrual cycle irregularities.

• Hypothyroidism can cause heavier, more frequent, or prolonged periods
• Hyperthyroidism can cause lighter or less frequent periods
• Other symptoms may include fatigue, unexplained weight changes, or heart palpitations

A simple blood test can check thyroid hormone levels, and appropriate medication can often restore a regular cycle once the underlying thyroid condition is treated.

Endometriosis

Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus — often on the ovaries or fallopian tubes. It’s a recognized cause of menstrual cycle irregularities and can involve:

• Abnormal or unpredictable bleeding patterns
• Severe cramping or pelvic pain before and during periods
• Pain during intercourse or bowel movements

Endometriosis can be difficult to diagnose since symptoms overlap with other conditions on this list. If period pain feels disproportionate to what’s typical for you, it’s worth raising with a doctor specifically.

Hormonal Birth Control

Starting, stopping, or switching hormonal contraception is one of the most common and often overlooked causes of menstrual cycle irregularities. This includes the combined pill, progestin-only pill, hormonal IUD, implant, patch, and injection.

• Breakthrough spotting is common in the first few months on a new method
• Hormonal IUDs and the implant can cause irregular bleeding or lighter periods for an extended period
• Cycles typically take a few months to settle after stopping hormonal contraception

This is usually not a cause for concern, but persistent irregular bleeding beyond the first few months on a method is worth mentioning at a follow-up appointment.