Why do I have irregular periods?

Katherine Maslowski

Periods, that one thing that happens to women around once a month, unless they’re irregular. Yep, not even the one thing that’s supposed to show up once a month can be totally reliable. You can have early periods, which can be annoying, or late periods which could be potentially stressful. So today we’re talking about irregular periods, why you might have irregular periods and when to seek advice about your periods.

What are irregular periods?

Periods are said to be irregular when the length of time between each period keeps changing. Your period might sometimes come early and sometimes late, it might be hard to predict when your next period is going to start.

Some medical terms:

Period = menses/menstruation
Irregular periods = oligomenorrhoea
Absent periods = amenorrhoea
Menstrual cycle = the cycle of hormones that occurs within your body to control your periods

Irregular periods are not always a sign that something is wrong, there are lots of reasons that women might have irregular periods. When girls first start getting their periods during puberty, they are often irregular and it can take several years for them to settle into a cycle. This is normal. Even among women who have “regular” periods it can be normal for there to be a few days of variation in how long each cycle is.

How to define your cycle

The day that you wake up bleeding is day 1 of your cycle and then you keep counting until the first day of your next period. The average cycle length for most women is 28 days but anything between 21 and 35 is considered “normal”. Some women have periods that are like clockwork – they come exactly on time, every month. Some women have reasonably predictable periods – they come at the same time every months within a few days. Other women have periods that are hard to predict and seem to come whenever they feel like it. This is called having irregular periods. The medical term for this is oligomenorrhoea.

The key players in the menstrual cycle

Source: https://teachmeanatomy.info/pelvis/female-reproductive-tract/vagina/

The ovaries are the organs that produce oestrogen and progesterone (and testosterone – see [insert link for the testosterone article on Selfish Darling] for more information about testosterone in women). These hormones then affect the lining of the uterus (womb). This lining is called the endometrium and it is this part that sheds as the blood when you have a period. The endometrium comes out through the cervix and the vagina during your period.

The ovaries are also where the egg is released during ovulation. The egg travels from the ovary through the fallopian tube and into the uterus. If it meets a sperm on its way fertilisation occurs which is the beginning of a pregnancy. If it doesn’t meet a sperm, the cycle continues.


The menstrual cycle and its hormones

The menstrual cycle is the name for the cycle of hormones that control your periods. There are four main hormones involved in this cycle:

FSH and LH are produced in your brain and travel through the blood to the ovaries. They then tell the ovaries to produce oestrogen and progesterone hormones. The fluctuations and the balance of these hormones over the course of the cycle is what leads to ovulation (the ovary releasing an egg). On average most people with regular periods will start bleeding about 14 days after ovulation.

Recommended read: Period Syncing and Sacred Bonding


What needs to happen for periods to be regular?

In order for periods to be regular, you need to be ovulating regularly. If ovulation is not happening, then the inside of the uterus isn’t getting clear hormonal signals about when it is time to bleed so the bleeding becomes erratic and hard to predict.

First, let’s look at what needs to happen for a period to be regular.

Source: https://www.britannica.com/science/menstruation/Hormonal-control-of-menstrual-cycle

The follicular phase

Let’s use a 28-day cycle as an example (but remember not everyone’s cycle follows this exact pattern). From day 1-14 is called the follicular phase and is when the body is preparing for ovulation and a possible pregnancy. The follicular phase is usually the part that varies in length between women.

  • The levels of oestrogen are increasing
  • The ovaries are getting eggs (follicles) ready to ovulate
  • The lining of the uterus (endometrium) is shedding during the period and then starting to grow again after bleeding stops

Day 14 is usually the “textbook” day of ovulation, however, it is important to note that not everyone ovulates on day 14, even if your cycle is exactly 28 days long.

The luteal phase

Between ovulation (day 14 in this example) and the first day of the next period is called the luteal phase. The luteal phase is USUALLY approximately 14 days long and is more consistent in length between women than the follicular phase.

  • The progesterone levels are rising
    • After ovulation, the structure left behind in the ovary is called the corpus luteum
      • This is what produces the progesterone
    • The endometrium is rearranging itself in preparation for a possible baby

Towards the end of the luteal phase, progesterone levels start to drop as the corpus luteum disappears and this is what tells the lining of the uterus that there is no baby and it is time to bleed.

What can “go wrong” in this process?

Periods become irregular when ovulation gets disrupted, so what disrupts ovulation?

The menstrual cycle is under hormonal control, so anything that disrupts the production of hormones can make periods irregular or make periods stop completely.


Causes of irregular periods:

Puberty = irregular periods in teens

    • This is the main cause of irregular periods in teenagers
    • Initially, the hormone cycle isn’t mature enough to reliably coordinate ovulation
    • It can take a few years after you first start getting your periods for them to settle into a cycle

Menopause = irregular periods in women over 45

    • When you start to go through menopause your periods might start to become irregular
    • This usually occurs between the age of 45 and 55
    • This is because your ovaries are not able to produce as much oestrogen as before
      • This means that there isn’t enough oestrogen to cause ovulation to happen
    • When your ovaries stop consistently producing enough oestrogen to trigger ovulation, this is when your periods stop altogether
      • Important: if it has been more than a year since your last period then you are “post-menopausal”. If you then start bleeding again, this needs to be checked out and you should discuss this with your doctor.


      • If your period is later than usual you might be pregnant and you may need to take a pregnancy test

Hormonal contraception

      • Sometimes being on hormonal contraception can cause irregular periods
        • This is most common with progesterone only pills (mini pill or POP), the implant, the contraceptive injection and hormonal intrauterine systems (IUS, such as Mirena, Kyleena or Jaydess)
      • Extreme weight changes, stress or excessive exercise
        • Significant weight loss or weight gain can disrupt your body’s production of hormones and can make your periods become irregular or even stop completely
        • Stress and excessive exercise can also affect your hormones and can make your brain stop producing the hormones required for the menstrual cycle to function
          • It can be helpful to think of your menstrual cycle like this in terms of stress/weight/exercise: in terms of biology, the purpose of the menstrual cycle is to allow a pregnancy. If your body is not in a state where it could sustain a pregnancy (not enough energy from food, too much energy spent on exercise, too stressed) then it is not going to bother “wasting time” on the functions that enable pregnancy
          • When people’s weight stabilises or the stress is resolved, their cycles will start to come back

Medical conditions

    • Polycystic ovarian syndrome (PCOS)
      • PCOS is a condition that affects how your ovaries work
        • NOTE: PCOS and polycystic ovaries (PCO) are different things. PCOS is not just about the ovaries, it can affect lots of parts of your body, however, PCO is simply having lots of small cysts on your ovaries
            • PCOS may cause irregular periods, difficulty getting pregnant, excessive hair growth, weight changes or acne
            • You can see these other pages on Hormona or the NHS website for more information about PCOS
    • Thyroid disorders
      • The different hormone systems within your body all interact with each other and so, an imbalance in another system can affect your periods as well
      • An underactive thyroid (hypothyroidism) can cause irregular or heavy periods
      • Other symptoms of hypothyroidism include depression, tiredness, constipation, weight gain, muscle aches/cramps, dry skin, brittle hair, loss of interest in sex or being sensitive to the cold
        • If you have these symptoms and you think you might have an underactive thyroid then speak to your doctor
        • You can see the NHS website for more information

When should I see a doctor?

If your periods have always been a bit irregular or you are still going through puberty then you don’t need to worry.

However, sometimes it is important to get things checked out by a doctor. There may not be anything “wrong” but is always a good idea to make sure.

It is a good idea to see your doctor if:

  • You are less than 45 years old, and your periods suddenly change to become irregular
  • Your periods come more frequently than every 21 days or less frequently than every 35 days
  • You bleed for more than 7 days in a row with your periods
  • You want to get pregnant but are struggling to do so and your periods are irregular
  • Your cycles vary so much that there is a large difference in length between the shortest cycles and the longest ones (more than 20 days difference)
  • You have bleeding in between your periods
  • You have bleeding after sex

If you need special tests to find out the cause of your irregular periods or you need treatment your GP may refer you to a gynaecologist.

NHS website has more information about when to see a doctor.

Does it matter if my periods are irregular?

If your periods are irregular but they do not bother you and your doctor has confirmed there is no concerning reason for them to be like this then you can leave them alone.

If you want to have a baby, it might be hard for you to get pregnant if your periods are irregular. This is because, in order to get pregnant, you need to ovulate. If you are not ovulating every month then your chance of getting pregnant is reduced. There are medicines that can help you ovulate if this applies to you and your GP can help with this.

If you have PCOS and you are not having more than 3 periods in a year, this can increase your chance of developing endometrial cancer (cancer of the lining of the uterus) in the future. It is a good idea to speak to a doctor about medicine to prevent this if this applies to you.

Katherine Maslowski

Katherine Maslowski

Katherine is a junior doctor from New Zealand who has experience working in Obstetrics and Gynaecology and is currently studying an MSc in Women’s Health. She is passionate about women’s health and empowering women to learn about their bodies and understand how they work. She is particularly interested in sexual and reproductive health and helping women to make educated, informed choices about their health and wellbeing.