Hypothetical example (not a real person’s data): period start dates logged over six months. Cycle length is counted from the first day of one period to the first day of the next.
| Period started | Next period started | Cycle length | Compared with the NHS range |
|---|---|---|---|
| 3 Jan | 2 Feb | 30 days | Within the 21 to 35 days the NHS describes as normal for a regular cycle |
| 2 Feb | 8 Mar | 34 days | Within the 21 to 35 days the NHS describes as normal for a regular cycle |
| 8 Mar | 5 Apr | 28 days | Within the 21 to 35 days the NHS describes as normal for a regular cycle |
| 5 Apr | 12 May | 37 days | Outside 21 to 35 days |
| 12 May | 9 Jun | 28 days | Within the 21 to 35 days the NHS describes as normal for a regular cycle |
In this example the cycles run from 28 to 37 days, a spread of 9 days. The NHS says regular cycles of 21 to 35 days are normal,[2] and describes irregular periods as a wide variation in the time between periods.[1] A log like this is also what the NHS suggests you take to your GP.[1] It shows a pattern; it does not diagnose a cause.
The NHS says irregular periods can be common during puberty and just before the menopause, and that changing contraception can disturb your cycle. Other causes include pregnancy, stress, sudden weight loss, overexercising, and conditions such as PMOS (previously called PCOS) and endometriosis.[1][3]
The NHS advises seeing a GP if your periods stop and you are concerned, or if period problems are affecting your daily life.[1]
Yes, the NHS lists stress among the common reasons for missed or stopped periods.[1]
Not necessarily, but pregnancy is not possible in a cycle where ovulation does not happen.[2] If you have been trying for a year or more, the NHS advises speaking to a GP.[4]
The NHS lists sudden weight loss among reasons periods can stop, and says weight that is too low or too high can make it harder to get pregnant.[1][4]
Your period arrives late. Then it comes early. Last month it lasted 3 days; this month it's dragging on for 8. If your cycle feels unpredictable, you're not alone — irregular periods are one of the most common reasons women visit their gynecologist, affecting up to 30% of women in their reproductive years.
But "irregular" covers a wide spectrum. A period that's a few days off occasionally is usually nothing to worry about. Cycles that are consistently unpredictable, very heavy, very painful, or missing entirely are telling you something more significant. Here's how to tell the difference — and what to do about it.
The NHS says regular cycles of 21 to 35 days are normal.[2] A cycle is irregular when there is a wide variation in the time between periods, the amount of blood lost or the number of days a period lasts.[1]
The NHS describes irregular periods as a wide variation in the time between your periods (they may arrive early or late), the amount of blood you lose, or the number of days the period lasts.[1]
This is probably the number one non-medical cause of cycle disruption. When you experience significant stress — emotional, physical, or both — your body releases cortisol and adrenaline. These stress hormones interfere with the hypothalamus's production of GnRH (gonadotropin-releasing hormone), which is the signal that triggers the hormonal cascade of ovulation. Without the normal GnRH pulse, LH and FSH don't surge properly, ovulation is delayed or suppressed, and your period follows suit.
A stressful exam period, a difficult breakup, a demanding work project, bereavement — all can cause your period to arrive late or skip entirely. Once the stressor resolves, cycles typically normalize within one to two months. We go deeper into this connection in how stress affects your menstrual cycle and fertility.
Both rapid weight loss and significant weight gain can disrupt your cycle. Body fat plays a direct role in estrogen production — too little body fat leads to insufficient estrogen, too much body fat leads to excess estrogen — both of which disrupt the hormonal balance needed for ovulation. Athletes with very low body fat, women recovering from restrictive eating disorders, and women who have gained significant weight rapidly are all at risk for menstrual irregularity.
High-intensity exercise — particularly when combined with caloric restriction and low body weight — can suppress the hypothalamic-pituitary-ovarian (HPO) axis and halt ovulation. This is known as the Female Athlete Triad (now broadened to Relative Energy Deficiency in Sport, or RED-S). Endurance athletes, competitive gymnasts, and dancers are particularly at risk. Reducing training intensity and increasing caloric intake usually restores cycles over time.
The NHS says irregular periods can be common during puberty and just before the menopause. Changing your method of contraception can also disturb your cycle.[1]
Your thyroid gland produces hormones that regulate metabolism — and these hormones are deeply intertwined with reproductive hormones. Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) can disrupt menstrual cycles. Hypothyroidism often causes heavier, more frequent periods; hyperthyroidism can cause lighter, less frequent periods or cessation. A simple blood test (TSH) can identify thyroid dysfunction, and treatment typically restores cycle regularity.
🩸 Key takeaway: Many causes of irregular periods are treatable once identified. Don't dismiss irregular cycles as "just how I am" — get evaluated.
PCOS (now also called PMOS) is one possible cause: irregular periods or long gaps between periods are among its main symptoms.[3]
Prolactin is the hormone responsible for milk production in breastfeeding women. When prolactin is elevated outside of breastfeeding (a condition called hyperprolactinemia), it suppresses ovulation and menstruation. This can be caused by a small benign pituitary tumor called a prolactinoma, certain medications (including some antidepressants and antipsychotics), or kidney and thyroid disease.
Endometriosis, where tissue similar to the womb lining grows outside the womb, can cause painful, heavy or irregular periods, pelvic pain and pain during or after sex.[1]
Sometimes called premature ovarian failure, POI occurs when the ovaries stop functioning normally before age 40. It causes irregular or absent periods and is associated with low estrogen and infertility. POI affects about 1 in 100 women and can be caused by autoimmune conditions, chromosomal abnormalities, or cancer treatment.
See your gynecologist if:
Stress is one of the things the NHS lists among reasons periods can stop or change.[1] Choose stress-management practices that work for you.
Make sure you're eating enough — particularly enough carbohydrates and healthy fats, which are essential for estrogen production and hormonal signaling. If you're restricting food or exercising intensely, this is often the first thing to address.
One of the most powerful things you can do is start tracking your cycle. Apps and our free Period Tracker tool can help you log your period start and end dates, symptoms, and flow. Over time, this data reveals patterns — and gives your doctor valuable information when you seek evaluation.
Don't rely on memory. Write it down or log it digitally, consistently. If you've been tracking for three or more months and your cycles are consistently irregular, bring that data to your appointment. It can accelerate diagnosis significantly.
Not all period irregularity is the same. Understanding which type you have helps identify the underlying cause:
We removed the table of irregular-period “types” with day-count definitions because we could not source it. See the NHS descriptions above.[1]
Some cycle variation is normal — but these situations warrant medical evaluation:
A basic hormonal panel (FSH, LH, estradiol, progesterone, thyroid, prolactin, androgens) usually provides significant insight into the cause and appropriate next steps.
Keeping a diary of your symptoms through the cycle can help. The NHS suggests doing this before you see your GP about period problems.[1]
This data is invaluable when you see a doctor — it transforms a subjective report of "irregular periods" into objective cycle history that guides investigation and diagnosis.
For many women, addressing lifestyle factors can meaningfully improve cycle regularity — often without medication:
Irregular cycles don't mean you can't conceive — but they do make timing more challenging. If your cycles vary significantly in length, predicting your fertile window from a calendar is unreliable. The most practical strategies:
Use our free Period Tracker and Irregular Period Checker to understand your cycle pattern.
Track My Period →Your menstrual cycle is more than a monthly inconvenience — it's a vital sign. Irregular periods are your body's way of telling you something needs attention. Taking that signal seriously, and getting the right support, is one of the most important investments you can make in your long-term health.