Irregular Periods: What Counts as Irregular and What Causes It

By Femly Health Editorial Team · 6 October 2026

Your periods are irregular if the gap between them is shorter than 21 days or longer than 35 days, or if the length of your cycle changes a lot from month to month. This is common and often not a sign of a problem, though sometimes it points to something worth checking with a doctor.

What Counts as an Irregular Period?

A menstrual cycle is counted from the first day of one period to the first day of the next. For most women, a normal cycle falls somewhere between 21 and 35 days. Some health guides use a slightly different range, saying a cycle shorter than 24 days or longer than 38 days counts as irregular.

Another way to spot irregularity is by how much your cycle length changes. If it varies by more than 20 days from month to month, that is considered irregular. A period itself usually lasts 2 to 7 days, with the heaviest bleeding in the first three days.

Doctors also look at other signs of irregularity, such as bleeding or spotting between periods, bleeding after sex, or not having a period for three to six months. If your periods stop completely, this is called amenorrhea, which simply means the absence of periods.

How Common Are Irregular Periods?

Irregular periods are common. Research suggests that somewhere between 14% and 25% of women have cycles that are shorter or longer than normal, heavier or lighter than usual, or come with other problems like cramps. In one study looking at women's menstrual patterns, this same 14% to 25% range was reported for how many women experience irregular cycles. So if your cycle is not perfectly regular, you are far from alone.

Irregular Periods During the Teen Years

If you are a teenager, or you're reading this for your daughter, irregular periods in the first few years after they start are normal. Most teens have cycles lasting between 21 and 45 days, a wider range than adult women. It often takes a few years for the body's hormone signals to settle into a steady pattern. This is a normal part of growing up, not something to worry about on its own.

Irregular Periods Approaching Menopause

At the other end of the reproductive years, cycles often become more irregular again. This time is called perimenopause, the years leading up to menopause when hormone levels start to shift and change. Menopause itself is defined as the point when periods stop permanently, confirmed after a full year without one.

During perimenopause, it is common for periods to come closer together, further apart, or with different flow than before. This is usually a normal transition rather than a sign that something is wrong, though any big or sudden change is still worth mentioning to a doctor.

What Causes Irregular Periods

Irregular periods can happen for many reasons. Hormones are often behind it. Shifts in the levels of estrogen and progesterone, the two main hormones that control your cycle, can throw off its usual pattern.

Some common causes include:

• PCOS, a condition where a hormone imbalance affects periods, ovulation, the ability to get pregnant, and metabolism
• Thyroid problems, both an underactive and an overactive thyroid
• Hormonal birth control, which can lighten, stop or change periods while you use it
• Breastfeeding and intrauterine devices
• Excessive exercise
• Eating disorders, including anorexia nervosa, bulimia nervosa and binge eating disorder

It's worth knowing that PCOS does not explain most cases of irregular periods. Research suggests only about 1 in 10 women with irregular cycles actually has PCOS, so many other causes are far more common. If you want to understand how PCOS and thyroid conditions can overlap and be confused for each other, this article on PCOS vs Thyroid: How These Two Conditions Are Connected covers it in detail.

Some factors that can be changed also play a role. These include being overweight, ongoing stress, and smoking, which can also raise the risk of periods stopping altogether.

When Irregular Periods May Need a Doctor's Look

Irregular periods are usually not a sign of a serious problem. But they can occasionally point to something that needs medical attention, so they shouldn't always be brushed off either.

Heavy periods are worth paying attention to. They are the most common sign of a bleeding disorder in women and girls. That said, having heavy periods does not automatically mean you have a bleeding disorder — most women with heavy periods do not. A doctor can help work out what is behind it.

Irregular periods have also been linked in research to a number of other health conditions, including metabolic syndrome, heart disease, type 2 diabetes, rheumatoid arthritis, anemia, osteoporosis, and lower quality of life. These are associations found in research, not proof that irregular periods cause these conditions, and it isn't clear whether they apply to all women with irregular cycles or only to some. In women who are or may become pregnant, irregular periods have also been linked to a higher chance of pregnancy-related high blood pressure disorders and other pregnancy complications.

Irregular Periods and Fertility

Irregular periods can make it harder to get pregnant. This is because your cycle is closely tied to ovulation, the release of an egg from your ovary.

An irregular cycle can be ovulatory, meaning ovulation still happens even though the timing is unpredictable, or anovulatory, meaning ovulation does not happen at all that cycle. Anovulatory cycles make it harder to know your fertile window, and if they happen often, they can affect your chances of conceiving. If you're trying to get pregnant and your cycles are unpredictable, tracking your cycle length and any other symptoms can give your doctor useful information.

What You Can Do Next

Start by tracking your cycle for a few months. Note the first day of each period, how long it lasts, how heavy the flow is, and any other symptoms like cramps or spotting between periods. This record is one of the most useful things you can bring to a doctor's appointment.

How irregular periods are approached often depends on the type of irregularity you have and personal factors, such as whether you are planning a pregnancy. There isn't one single fix that works the same way for everyone, which is why a proper conversation with a doctor matters.

Alongside medical care, lifestyle factors like sleep, stress and movement can play a role in supporting your overall hormonal wellbeing. Femly Health's programmes, such as the PCOS Care Program, offer lifestyle-led guidance and accountability to support women managing symptoms like irregular cycles, always alongside a doctor's care rather than instead of it.

If your periods change suddenly, become very heavy, stop for several months without explanation, or come with pain that worries you, it is worth booking a doctor's appointment. Keeping a simple period diary and talking openly with your doctor is often the most useful first step.