HA_Irregular_Periods_02For many women, that ‘time of the month’ isn’t actually a monthly occurrence at all.

In fact, about one third of women do not have regular periods. This can mean that the time between periods is very long, or changes from cycle to cycle.

Having irregular periods isn’t always a bad thing, but it can definitely be a nuisance. Like when it arrives when you’re unprepared, or when you are planning to go on holiday.

More seriously though, irregular periods add a degree of difficulty to getting pregnant or can be the first signal that something else may be wrong with your health.

So how do you know if your periods are normal? What is a normal length of time between periods? And how irregular can they be before you should start to worry?

What is a normal period?

When doctors describe a “normal” period as 28 days, they are referring to what happens on average for most women. This means if you ask a large number of women how long it is between the first day of one period to the first day of the next, 28 days lies roughly in the middle. But women are different, and the length between periods can take anywhere between 21 and 40 days.  

Periods are influenced by the hormones estrogen and progesterone which rise and fall at different times across the cycle. From the beginning of the cycle, estrogen levels rise progressively, which occurs as an egg in the ovary matures. An egg is then released from the ovary – this is called “ovulation”.  Estrogen levels peak at ovulation.

The time from the period to ovulation is what varies between women, and also can vary from cycle to cycle if the time between each of your periods is different. Once ovulation has occurred, the time to the next period is very constant between women – usually 13-14 days.

Progesterone levels increase only after ovulation has occurred. This helps the uterus (womb) prepare for pregnancy. During this time, the lining of the uterus changes, so that a fertilised egg can embed and grow into a baby. If pregnancy does not occur, the lining isn’t needed and is shed at the end of each cycle. Progesterone levels start to fall around 12 days after ovulation, which triggers a period.

Periods can last anywhere from 2-7 days and can differ in length from one cycle to the next.

The “heaviness” of the period depends on how thick the lining became during the cycle. The lining thickness is mostly a reflection of how high the estrogen level was across the cycle. It is also sometimes influenced by how long it has been since the last period. Sometimes with longer cycles the lining becomes thicker, as it has been exposed to estrogen for a longer time before ovulation.

Understanding the length of your cycle

HA_irregular_periodsTo get an idea of your cycle length and when your periods are due, it is a great idea to keep a record of your periods. This can be done on a period tracker app on your phone or on a calendar.

The length of a cycle is counted from the first day of one period to the first day of the next.

It is a good idea to track your period length over several months to get an idea of how much your cycle changes.  This can help you find out what is “normal” for you. If you are ever concerned and decide to see a doctor, this record also makes it easier to explain what is happening with your cycles, particularly if there is no observable pattern.

Why does my period change colour?

Periods can vary in colour, from bright red to very dark brown/black. Blood that is shed quickly is generally much brighter than blood that has been sitting around for longer. This is why brighter blood is often seen at the beginning of a period, while darker blood generally occurs towards the end. It is also not unusual for light periods (where less blood shed) to be darker than heavier periods. This is because the colour of blood is altered as it flows more slowly. Small blood clots are generally no reason for concern, as blood naturally clots when it sits around for a while in the uterus before it is shed.

So, should I worry about “irregular” periods?

According to doctors that specialise in hormones (endocrinologists), there is generally no need to worry about a few irregular periods a year. This is because not every woman runs like clockwork. In fact, about one in three women report having irregular periods, making irregular periods quite normal.

As a general rule, doctors recommend having a check-up if:

  • You have less than 8 periods a year
  • You go three months without a period
  • Your periods happen less than 21 days apart
  • Your periods last longer than 8 days

But this may also just be how your system works. Seeing your doctor can help figure this out.

If you notice unusual changes to your period, it is also a good idea to see your doctor, as treatment may be needed. This can include if you notice:

  • A high or unusual amount of blood clotting in your period, or
  • Your periods have an unusual texture and/or smell.

What can cause missed periods?

The most common reasons for missing periods are being pregnant, using hormone contraceptives (like the oral pill) and menopause.

Periods are often irregular during the first few years after your periods start, and again in the years before the menopause. They can happen more or less often, or be lighter or heavier, than usual.

Periods can also be irregular as a result of imbalanced hormones. There are a number of reasons why hormonal imbalance may occur. This can be due to a sudden change in weight (both in increase and a decrease in weight), which may occur with a change in your exercise levels or with a change in your eating patterns. Interestingly, for some women losing weight and starting exercise may make your periods come back more regularly.

Stress and being unwell can also cause you to miss a period, particularly when the stress occurs in the interval between a period and when you would normally ovulate.

Women who have eating disorders, such as anorexia nervosa and bulimia, also will stop having regular periods usually associated with a change in weight.

When hormones become imbalanced, periods can stop altogether. This is called “amenorrhea” or no periods. Regular periods can return with lifestyle changes, although this can take some time and be difficult. Your doctor (GP) can help you find a specialist (endocrinologist) to support you through these changes.

Irregular periods may also be the first signal that you have a condition known as polycystic ovarian syndrome (PCOS). This is one of the most common conditions in reproductive aged women, and can affect women in different ways1. Some women with polycystic ovaries can have regular cycles but cycles are more often irregular and unpredictable or have become so over time. PCOS may or may not be associated with other physical changes like gaining weight, developing unwanted hair, or losing more scalp hair than usual. Your doctor can do blood tests and sometimes also an ultrasound to see if you have polycystic ovaries.

If your period is irregular, your doctor may also measure the level of your thyroid hormones and your prolactin levels, as changes in these hormones can cause periods which were previously regular to become irregular.  

So, if you have fewer than 8 periods a year, or go three months without a period, go to your doctor to figure out why. This is important, as your doctor can help with a plan to get your periods and your hormones back on track.  

The Bottom Line

All women are made differently.  About one third of women have irregular periods, which actually means that having irregular periods is fairly normal.  But if you do have fewer than 8 periods a year, go three months without a period or are concerned, it is a good idea to see your doctor (GP).

Author Information

HA_Nicolette_HodylNicolette Hodyl PhD is the Director of Vertex Health, a consultancy specialising in health research strategy and health communication. She is the Head Writer for Hormones Australia and has over 15 years’ experience working in medical research.

Expert Review

This information has been medically reviewed by Associate Professor Jane Holmes-Walker, PhD MBBS FRACP, who is a Senior Staff Specialist in the Department of Diabetes and Endocrinology at Westmead Hospital.


  1. Teede H, Misso M, Costello M, Dokras A, Laven J, Moran L, et al. International evidence based guideline for the assessment and management of polycystic ovary syndrome. Melbourne Australia: Monash University; 2018.