Why your cycle went irregular, 8 possible reasons
Irregular periods usually mean ovulation has become unreliable. The most common causes are thyroid dysfunction, PCOS, significant stress or under-eating, weight change, coming off hormonal birth control, perimenopause, elevated prolactin, and certain medications or illnesses. A cycle consistently shorter than 21 days or longer than 35 needs investigation.
First, what counts as irregular
A normal cycle runs roughly 21 to 35 days, measured from the first day of bleeding to the day before your next period starts. Some month-to-month variation is expected. Irregular means the pattern itself is unpredictable, or your cycles sit consistently outside that range.
The eight
1. Thyroid dysfunction. Both underactive and overactive thyroid disrupt cycles, and it is one of the simplest things to test. Often overlooked despite being straightforward to check.
2. PCOS. Polycystic ovary syndrome is a leading cause of irregular or absent ovulation. It frequently comes with acne, hair changes and insulin resistance, though presentation varies widely and you do not need visible cysts to have it.
3. Stress and hypothalamic suppression. Sustained physical or psychological stress can suppress the signalling from your brain that drives ovulation. Your body deprioritises reproduction when it reads the environment as unsafe.
4. Under-eating or over-exercising. Related to the above and extremely common in women who consider themselves healthy. Insufficient energy availability, even without being underweight, can switch ovulation off. Missing periods while training hard is a signal, not an achievement.
5. Weight change. Significant gain or loss in either direction affects estrogen production and cycle regularity, since fat tissue is hormonally active.
6. Coming off hormonal birth control. Cycles commonly take three to six months to establish a pattern. If you were irregular before starting, that pattern often returns, because the pill managed it rather than resolved it.
7. Perimenopause. Usually from the late 30s onward. Cycle length changing by seven days or more is often the first sign, and this can start earlier than most people expect.
8. Elevated prolactin, medications and illness. High prolactin interferes with ovulation. Some antidepressants, antipsychotics, thyroid medication and steroids affect cycles, as do periods of significant illness.
When to get it checked
No period for three months or more
Cycles consistently under 21 days or over 35
Bleeding between periods, or after sex
Sudden change from a pattern that was previously reliable
Irregularity plus other symptoms: hair loss, new facial hair, nipple discharge, significant fatigue, unexplained weight change
What to bring to the appointment
Three months of tracked cycle data: start date, length, flow, symptoms. Then ask the direct question: "Can we check thyroid and prolactin, and can we look at whether I'm ovulating?" Specific requests are harder to wave away than general concern.
Where a blend fits
Shift was formulated for irregular cycles and the post-birth-control transition. It is support alongside investigation, not a substitute for finding out why.
This article is educational and not medical advice. Absent or persistently irregular periods should be assessed by a healthcare provider.
