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What’s causing my irregular periods?

Irregular periods can feel worrying, especially when the reason for irregularity doesn’t seem clear. With the menstrual cycle sometimes being described as the ‘fifth vital’ sign of health, unexpected changes in your periods may hint that something else is going on behind the scenes.

There are many reasons that you may be experiencing irregular periods, keep reading to find out more.


  • What is the menstrual cycle?

  • What are irregular periods?

  • What can cause irregular periods?

  • How are irregular periods treated?

Period products on a pink background

What is the menstrual cycle?

The menstrual cycle is a female hormonal cycle that typically ranges between 23-35 days (1).

Hormone levels change throughout the menstrual cycle, causing the uterus (womb) lining to change. The menstrual cycle can be split into 3 main phases, with hormonal differences determining the stage.

1.Follicular phase: This starts on day one of your period. The thickened uterus lining breaks down, due to there not being a fertilised egg to support. During this phase, follicle stimulating hormone (FSH) is produced. This causes a follicle in the ovary to produce a mature egg. As the egg matures, the hormone oestrogen increases.

2. Ovulation: When oestrogen levels start to peak, the body produces luteinising hormone (LH). This causes the follicle in the ovary to release the egg - this is ovulation. Oestrogen levels decline after ovulation. Ovulation usually occurs 10-16 days before the next period (1).

3. Luteal phase: This lasts from ovulation until the start of the next period. The empty follicle becomes a corpus luteum, which releases the hormone progesterone, and a small amount of oestrogen. Progesterone helps the uterus lining to continue thickening, in preparation for a potential pregnancy. If pregnancy doesn’t happen, the corpus luteum stops releasing progesterone, triggering a period to start, and the return to the menstrual phase.

If pregnancy does occur, the corpus luteum continues to release progesterone to maintain the uterus lining. 

What are irregular periods?

The menstrual cycle typically repeats itself, meaning that a woman can expect to experience her period at the same time each month or in her cycle. Given that a ‘healthy’ menstrual cycle can range between 23-35 days, some women may not experience periods once each month.

Irregular periods are defined as having a menstrual cycle that is less than 21 days or more than 35 days (2). However, it’s important to note that in the year following the first period, it can be common to experience irregular periods.

Amenorrhea is the scientific term used to describe the absence of a period, in women of reproductive age (3).

Keeping track of menstrual cycles can help with identifying irregular periods. You can do this manually, or there are many apps available, such as Clue or Flo.

Woman tracking her period on an app

What can cause irregular periods?

Irregular periods are usually due to hormonal imbalances in the menstrual cycle (3). This may mean that ovulation (the release of an egg from the ovaries) isn’t happening, and therefore further hormonal changes cannot occur to ‘trigger’ a period.

However, there may also be some physiological reasons for irregular periods.

Below are some of the common causes of irregular periods.


If you are sexually active, and your period is late, it’s worthwhile taking a pregnancy test. 

Polycystic ovarian syndrome (PCOS)

PCOS is a common hormonal condition that has metabolic and reproductive effects. It can result in irregular or missing periods. This can be due to an imbalance in hormone levels that prevents ovulation from occurring. 

Symptoms of PCOS include acne, excess hair growth in places like the face, chest and back, hair loss or thinning on the scalp, weight gain, insulin resistance, and difficulty getting pregnant.

Thyroid problems

If the thyroid gland is producing too much or too little thyroid hormone, this can cause irregular periods.

Hypothyroidism occurs when not enough thyroid hormone is produced, and symptoms can include weight gain, feeling cold, a slow heart rate and heavier periods (4).

Hyperthyroidism happens when too much thyroid hormone is produced. Symptoms can include weight loss, heart palpitations and lighter periods (4).

Hormonal contraceptives

The NHS states that some hormonal contraceptives can cause irregular bleeding during the first few months of starting (5).

Once hormonal contraceptives have been stopped, such as the combined oral contraceptive pill, it can take the body time to readjust before periods become regular. 

If you are concerned about anything in relation to hormonal contraception, or medication in general, speak with your doctor.


Perimenopause occurs before menopause. During this time, reproductive hormone levels fluctuate and this can result in irregular periods. Periods may become lighter or heavier, and may be more or less frequent.

Menopause is marked by 12 months of no periods. Whilst the average age of menopause is 51 years in the UK, women of any age can experience menopause. This is called primary ovarian insufficiency (POI).

Hypothalamic amenorrhea

The hypothalamus, which is located in the brain, is responsible for producing gonadotropin-releasing hormone (GnRH) which interacts with FSH and LH in the menstrual cycle. Disruptions to the hypothalamus may cause periods to stop or become irregular. This is known as hypothalamic amenorrhea.

This can be caused by undereating, overexercising, high stress levels and having a low body weight (i.e. a BMI below 18.5).


Periods may not become regular until breastfeeding has significantly reduced or stopped altogether. This is due to the hormone prolactin, involved in breastmilk production, which disrupts ovulation (6,7).

How are irregular periods treated?

Treatment for irregular periods will depend on the cause.

Speak to a doctor about your irregular periods. They will be able to investigate further. 

For example, they may investigate PCOS, which can involve having an ultrasound of the ovaries, and having blood tests to assess androgen hormone levels. They may check thyroid hormone levels too via a blood test, to assess for thyroid problems.

Your doctor may give you the option for medication or other medical interventions, dependent on the cause of your irregular periods.

You may wish to explore how diet and lifestyle can help regulate periods. In those with PCOS, dietary and lifestyle changes can be effective at controlling symptoms, including regulating periods. If thyroid problems or hypothalamic amenorrhea are diagnosed, your doctor may advise you to seek support from a Registered Nutritionist or Dietitian, in addition to medical support. Dietary and lifestyle changes cannot prevent perimenopause, but they may help look after overall health and improve quality of life.


Tracking menstrual cycles will help to identify irregular periods. Pregnancy, PCOS, thyroid problems, hormonal medication, perimenopause, hypothalamic amenorrhea, and breastfeeding are some of the common causes of irregular periods.

Treatment for irregular periods will be dependent on the cause. Speaking to a doctor will enable you to identify the right route for treatment, which may involve medical interventions and dietary and lifestyle changes. 

Further Support

If you’ve been diagnosed with PCOS or are struggling with irregular periods and have been advised to seek support with your diet and lifestyle, you can book a complimentary 15 minute discovery appointment with Lucy Jones ANutr, Founder of Lutrition. In this appointment, you’ll be able to explain what you’re experiencing, and together we can outline a plan of action for you. Book here.



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