Irregular Periods: What They Are and Why Cycle Tracking Still Matters

An irregular cycle doesn't always mean something is wrong. For many women, variations in cycle length or flow are a normal part of life at certain stages. Teenagers, for instance, often experience unpredictable cycles for several years after their first period, as the hormonal system is still maturing and establishing its rhythm. Women in perimenopause, the transitional phase leading up to menopause, similarly experience increasing irregularity as oestrogen and progesterone levels naturally fluctuate and decline. In both cases, some degree of cycle variation is expected and not a cause for concern on its own.

That said, irregular periods can sometimes signal an underlying condition that warrants attention. Understanding the difference between normal variation and a pattern worth investigating is where cycle awareness becomes genuinely valuable.

What Are Irregular Periods?

A typical menstrual cycle lasts between 21 and 35 days, with periods lasting four to seven days. The average cycle length is around 29 days. When cycles fall outside these ranges or when bleeding patterns change significantly this is generally considered irregular. Irregular periods can be caused by hormonal shifts, stress, underlying health conditions, medications, and other factors.

What Counts as Irregular?

Some variation from cycle to cycle is completely normal. However, the following patterns are generally considered irregular:

• Cycles shorter than 21 days or longer than 35 days
• Missing three or more consecutive periods
• Bleeding that is significantly heavier or lighter than your usual flow
• Periods lasting longer than seven days
• Cycle length varying by more than nine days from one month to the next
• Severe pain, cramping, nausea or vomiting during your period
• Spotting or bleeding between periods, after sex, or after menopause
• Soaking through a pad or tampon within an hour

It's worth noting that you don't need to predict your cycle to the exact day for it to be considered normal. Slight variations in length or flow are common and don't necessarily indicate a problem.

Related Conditions

Several recognised conditions are associated with irregular menstruation:

• Amenorrhoea: the complete absence of periods. Going 90 days or more without a period is considered abnormal unless you are pregnant, breastfeeding, or going through menopause (typically between ages 45 and 55). Not having started menstruating by age 15 or 16 may also indicate amenorrhoea.
• Oligomenorrhoea: infrequent periods, where cycles are longer than 35 days or you have fewer than eight periods per year.
• Dysmenorrhoea: the medical term for painful periods and significant menstrual cramping. Some discomfort is normal, but severe pain is not.
• Abnormal uterine bleeding: bleeding between periods, prolonged bleeding, or unusually heavy flow.

What Causes Irregular Periods?

Irregular cycles can stem from a wide range of causes, from everyday stress to underlying medical conditions.

Health Conditions Associated with Irregular Periods

• Endometriosis: a condition where tissue similar to the uterine lining grows outside the uterus, often on the ovaries or fallopian tubes. It can cause abnormal bleeding, cramping, and significant pain around menstruation.
• Pelvic inflammatory disease (PID): a bacterial infection affecting the female reproductive system, often resulting from an untreated sexually transmitted infection. Symptoms can include irregular periods, pelvic pain, and unusual vaginal discharge.
• Polycystic ovary syndrome (PCOS): a metabolic condition in which the ovaries produce elevated levels of androgens, which can delay or prevent ovulation and lead to irregular or absent periods.
• Primary ovarian insufficiency: occurs when the ovaries stop functioning normally before age 40, leading to missed or irregular periods. It can be associated with certain cancer treatments or autoimmune conditions.
• Thyroid disorders: both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) affect hormone regulation and can disrupt the menstrual cycle. See below for a deeper look at why thyroid health deserves more attention in the context of irregular periods.
• Bleeding or clotting disorders: can contribute to unusually heavy menstrual bleeding.
• Uterine or ovarian cancer: certain cancers can affect menstrual patterns, including causing heavier-than-usual bleeding or missed periods.

The Thyroid Connection: An Overlooked Cause of Irregular Cycles

Of all the conditions linked to irregular periods, thyroid dysfunction is arguably the most underdiagnosed. It's frequently missed, or dismissed, in women who are told their cycles are simply "a bit irregular" without further investigation. Yet the thyroid's influence on menstrual health runs deeper than most people realise.

When thyroid function is impaired, the effects on the reproductive system are far-reaching. Poor thyroid function raises prolactin levels, a hormone that when elevated directly suppresses ovulation. It also compromises insulin sensitivity, which increases the likelihood of developing PCOS-like hormonal patterns. But perhaps most significantly, thyroid dysfunction depletes the cellular energy available to the ovaries at a time when they need it most.

Ovulation is not a simple or quick process. A follicle takes around 100 days to mature from its earliest stage through to the moment of ovulation. Once released, the body must then rapidly form a corpus luteum, a temporary structure that can grow to around 4 cm in a single day, and produce enough progesterone to support the luteal phase. This is an enormous metabolic undertaking. When thyroid function is low and cellular energy is compromised, the ovaries simply may not have the resources to complete this process reliably, resulting in delayed ovulation, anovulatory cycles, or irregular periods that can be difficult to explain through other means.

Basal Body Temperature tracking can also offer an early clue. Women with hypothyroidism often have a consistently lower waking temperature than expected, which can be visible in their BBT chart before a formal diagnosis is made. If your temperatures are running persistently low and your cycles are irregular, it's a pattern worth discussing with your GP alongside a thyroid function test.

If you have irregular cycles and haven't had your thyroid checked, it's worth raising with your GP, particularly if you also experience fatigue, feeling cold, hair thinning, or difficulty losing weight.

Why Cycle Tracking Matters When Your Periods Are Irregular

Tracking your cycle, particularly with a device that measures Basal Body Temperature, gives you something far more useful than a calendar prediction: it tells you what your body is actually doing.

One of the most important distinctions in irregular cycles is whether ovulation is occurring at all. A long cycle is not necessarily a problem if ovulation simply happened later than usual. A delayed ovulation is very different from an anovulatory cycle, where no egg is released. Without temperature data, the two can look identical from the outside. With BBT tracking, the post-ovulatory temperature rise confirms whether ovulation occurred, and when.

This kind of information is valuable in many situations: for women trying to conceive who need to know their actual fertile window, for those managing PCOS or thyroid conditions who want to monitor whether treatment is restoring ovulation, and for anyone who simply wants to understand their body rather than guess at it. Devices like Daysy and Lady-Comp are specifically designed to work with irregular cycles, using a clinically validated algorithm that adapts to your individual pattern rather than assuming a textbook 28-day cycle.