Menstrual Cycle Basics: What You Need To Know-image

Menstrual Cycle Basics: What You Need To Know

When it comes to our menstrual cycles, usually it’s our periods that get all the air-time. But the menstrual cycle is actually made up of two separate cycles that each run from bleed to bleed (who knew?). Read on to learn everything you need to know about your cycle.


Key takeaways:

  • Your menstrual cycle is way more than just your period. It’s the time between the first day of your bleed and the first day of your next bleed.
  • Your menstrual cycle is carefully regulated by your hormones—namely oestrogen, progesterone, follicle-stimulating hormone (FSH) and luteinising hormone (LH).
  • The menstrual cycle is split up into the ovarian cycle, controlling the release of your eggs and the uterine cycle, controlling the build-up and shedding of your womb lining.
  • The first half of your cycle, the follicular phase, helps prepare your chosen egg to be released from this month’s ovary and develop a new womb lining after your period.
  • The second half of the cycle, the luteal phase, helps prepare the uterus for pregnancy or, if pregnancy doesn’t occur, to start the next cycle by triggering your period.

‘On the blob?’ ‘Got the painters in?’ Limited sex education, cultural stigmas and a healthy dose of sexism are to thank for the taboos we all face around ‘that time of the month’.

Really it’s no surprise that many of us have grown up knowing very little about our bodies, let alone understanding the ups and downs of our menstrual cycles

The American College of Obstetricians and Gynecologists (ACOG) has now classified the menstrual cycle as a vital sign—putting it right up there with your heart rate and blood pressure.

So yeah, your menstrual cycle is a big deal. Period.

Not only is it obviously super important for your fertility, but it also acts as a looking glass for your overall health. That’s because it can very easily be influenced by other factors such as underlying health conditions, stress, diet, weight fluctuations, exercise, sleep, illness and certain medications (we could go on… but you get the picture).

Understanding and being in tune with your cycle will put you in control of your health and give you agency over your body and wellbeing. 

So, without further ado, let’s take a deep dive into everything menstrual health—from its various phases to how to know whether your cycle is normal or not.


What is the menstrual cycle?

Your cycle starts on the first day of your period (when you bleed) and runs all the way through until the first day of your next period. Your cycle is carefully regulated by your incredible hormones—namely oestrogen, progesterone, lutenising hormone (LH) and follicle-stimulating hormone (FSH) (but more on those later).

Your menstrual cycle kicks in around puberty and your first period is called menarche. Unless you get pregnant or have a medical issue which affects your periods, you will continue to experience menstrual cycles throughout your reproductive life until menopause.

The average age of the first period is about 12, while the average age of menopause in the UK is 51. That’s approximately 468 periods you’ll have in your lifetime!

The menstrual cycle is made up of both the ovarian cycle and the uterine cycle.

During the ovarian cycle, one of your ovaries will develop an egg which will be released mid-cycle, in the hope that it will meet an eligible sperm, resulting in fertilisation and then a pregnancy. 

Whilst your ovaries are busy prepping and releasing this month’s egg, the uterine cycle is hard at work building up your endometrium (womb or uterine lining) in preparation for a fertilised egg. 

If your egg doesn’t meet its desired sperm, your hormones will say the word and your endometrium will be shed, resulting in a period, and the whole process starts again.

Let’s dig a little deeper into the ovarian and uterine cycles…

What happens during the ovarian cycle?

The ovarian cycle includes three main phases, the follicular phase, ovulation (when an egg is released from the ovary) and the luteal phase.

The Follicular Phase

Day 1 of your period is counted as day 1 of your menstrual cycle. It is also when the follicular phase begins—extending right through until mid-cycle, around ovulation. For most people, this is around 10-16 days, but it can vary from cycle to cycle. This is usually the main reason why your cycle length may vary month to month. 

The main job of the follicular phase is rallying up your immature eggs within the ovary to start their adulthood maturation process and to prep the chosen egg for release into the big wide womb.

Firstly, gonadotrophin-releasing hormone (GnRH) is secreted from your brain, which kick-starts and controls the following cascade of events.

This promotes the release of another hormone—follicle-stimulating hormone (FSH) from the brain. FSH then (you guessed it) stimulates your follicles, which are sacs within your ovaries containing immature eggs, and a few selected follicles will begin to grow and mature.

Did you know?

Women and gender-diverse people assigned female at birth are born with all the eggs they will ever have. This pool is called the ovarian reserve

As the follicles grow, they begin to produce what we like to call the matriarch of female sex hormones— oestrogen. Oestrogen is the hormone that makes you look and feel fabulous. She makes our hair and skin glow and gives us that spring in our step. This is why around this time in your cycle, you’ll likely be feeling your most confident, sexy and unstoppable.  

Around day eight of your cycle, one follicle will be picked as the chosen one and the others will stop maturing. This chosen one is called the dominant follicle and is given its shot at living out its dreams to go and find that eligible sperm.

At this point, there is enough oestrogen in your body to kick-start the next phase of the cycle—ovulation.

High levels of oestrogen trigger the brain to produce a hormone called luteinising hormone (LH). A sudden surge in LH levels triggers ovulation, which is when the now mature egg will be released from your ovary, where it starts its journey down the fallopian tubes, on route to the womb.  

Ovulation takes place 28-36 hours after the onset of the LH surge and 10-12 hours after LH reaches its peak.

You generally hear that ovulation happens on day 14, however, this is based on the assumption that everyone has a 28-day cycle, which is not always the case. But generally speaking, ovulation usually happens around the middle of your cycle, if you’re having regular periods. You can read more about ovulation and the signs of ovulation in our Knowledge Centre.

Did you know?

The highest chance of conceiving is during a six-day period called the fertile window.

It includes the five days leading up to ovulation and the day of ovulation. Although the egg can only survive about 12-24 hours after ovulation if not fertilised, the sperm can survive up to 5 days in the reproductive tract.

While on its tour around the reproductive system, if the egg happens to meet a sperm that takes its fancy in the Fallopian tube, they might choose to unite, forming an embryo. The embryo will try and set up camp in the innermost lining of the uterus, the endometrium, during a process called implantation.

Did you know?

You can get your period even though you may not have ovulated during that cycle. It is called an anovulatory cycle. Having the occasional anovulatory cycle is normal, however, it is important to know you cannot get pregnant during this type of cycle.

Underlying health conditions that impact your hormones, such as polycystic ovary syndrome (PCOS) or a thyroid hormone imbalance such as hyperthyroidism or hypothyroidism can cause irregular ovulation, which can make trying to conceive difficultly. Detecting PCOS or thyroid issues is easy to do via one of our specialised hormone testing kits.

The Luteal Phase:

This egg-scape leads into the next phase, the luteal phase, which extends from the day after ovulation to the day before your next period. The duration is usually the same in each cycle and lasts about 14 days.

The empty follicle left behind will get converted into a structure called the corpus luteum. It primarily produces a hormone called progesterone, which is important for pregnancy, and small amounts of oestrogen. Progesterone thickens the endometrial lining making to help support the growth and development of the embryo.

If a sperm-meet-egg moment happens and fertilisation occurs, the corpus luteum will keep growing until the placenta (this is the organ that develops to help the baby get its food and oxygen from mum) can take over the job of producing progesterone. 

However, if fertilisation does not happen, the corpus luteum will shrink causing a drop in both progesterone and oestrogen levels triggering your period.

If you don’t get pregnant, it’s common to feel a little low and languid during this phase of your cycle. This is because progesterone is literally preparing you for pregnancy, prompting you to slow down and nestle in. This is also the phase during which premenstrual syndrome (PMS) might rear its ugly head.


What happens during the uterine cycle?

The uterine cycle includes all the changes happening in the endometrium as it prepares to be the perfect comfy home for the potential baby-to-be.  

The menstrual phase: 

This is when you’re menstruating or having your period.

If there’s no sperm-meet-egg moment and fertilisation doesn’t occur, falling oestrogen and progesterone levels will trigger the breakdown of the endometrium because it’s no longer needed to support a pregnancy. It’s then shed along with menstrual blood and mucus through your vagina during your period. 

An average period length is 3-5 days, but a normal period is classified as lasting anything between 2 and 7 days, with the heaviest bleeding usually during the first two days.

The length of your periods can vary from cycle to cycle because it’s easily influenced by any factors that impact cycle length. 

Consistently irregular periods are characteristic signs of underlying health conditions such as PCOS, premature ovarian insufficiency (POI) and hormonal imbalances influencing thyroid, oestrogen or testosterone levels. 

Did you know?

The pain you experience during your periods is because of a chemical called prostaglandin which promotes contraction of the uterus so the endometrium can be shed.

Although some pain during your periods is normal, excessive period pain that stops you from going about your daily routine is not and is one of the signs of a condition called endometriosis.


The Proliferative Phase:

After your period ends, the proliferative phase starts. Increasing oestrogen levels during the first phase of your menstrual cycle prompt your endometrium to grow back to a thick, blood vessel-rich tissue lining ready to become a suitable home for the embryo.


The Secretory Phase:

After ovulation, your endometrium levels up into the secretory phase. Under the influence of progesterone, your endometrium will then start to secrete important chemicals in preparation for implantation and pregnancy.

If implantation occurs, your endometrium will stay thick, receiving increased blood flow to support the baby-to-be.  

If implantation doesn’t occur, the corpus luteum will dissolve, causing a decline in progesterone and oestrogen levels, triggering your period and starting the cycle all over again.

If you are experiencing any symptoms or want to understand where your fertility and reproductive health are currently at, we’re here to get you the answers you deserve. The Hertility Health journey includes an online health assessment, tailored at-home hormone and fertility test, an in-depth gynaecology report, fertility & gynaecology triage service, fast-tracked access to specialist clinics, and educational content and a community of 1000s of women. We’re here to support you throughout your reproductive journey, from menstruation to menopause.


Okay, but how do I know my cycle is normal?

Although we were often told growing up that the menstrual cycle is 28 days, we now know that the majority of people don’t have a 28-day cycle. A ‘normal’ menstrual cycle lasts between 21 and 35 days, but cycle length varies from person to person and sometimes even from cycle to cycle. 

Having a one-off longer or shorter cycle is usually nothing to be worried about. However, if you find your cycles constantly falling out of this range, it might be your body trying to tell you something is not quite right. 

Your cycle length may also change over time. Usually, your cycle is irregular during the first few years after menarche, becoming regular during your 20s, and then irregular again during perimenopause (the lead-up to menopause). 


How do I know if my menstrual cycles or periods are irregular? 

The best way to figure out if your periods are regular or not is to track your menstrual cycles. You could use an app or simply note it down in a journal. Your periods are defined as irregular when your periods are consistently shorter than 2 days or longer than 7 days.  

On the flip-side, your menstrual cycles are defined as irregular when they are consistently shorter than 21 days or longer than 35 days, or the difference between your shortest and longest cycle is longer than 20 days.


Want to understand your cycle, hormonal symptoms or irregular periods? 

Well, luckily you’re in the right place. At Hertility, we can help you get to the bottom of any hormonal symptoms you may be experiencing and give you clarity on whether a hormonal imbalance may be impacting things like irregular periods. We can also help you learn about your egg quality if you’re thinking about trying to conceive now or in the future. 

Check out our Hormone and Fertility Testing Kits to find out more. 



Angela R. Baerwald, Gregg P. Adams, Roger A. Pierson, Ovarian antral folliculogenesis during the human menstrual cycle: a review, Human Reproduction Update, Volume 18, Issue 1, January/February 2012, Pages 73–91, 

Reed BG, Carr BR. The Normal Menstrual Cycle and the Control of Ovulation. [Updated 2018 Aug 5]. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Endotext [Internet]. South Dartmouth (MA):, Inc.; 2000-. Available from: 




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