Menstrual Cycle 101: Everything You Need to Know About Your Cycle-image

Menstrual Cycle 101: Everything You Need to Know About Your Cycle

Medically Reviewed by Hertility on March 26, 2024

The menstrual cycle is an incredibly important process that governs female fertility and can be a signifier of your overall health. The menstrual cycle is made up of two separate cycles that each run from bleed to bleed. Here’s everything you need to know, from your period to the proliferative phase. 

Quick facts:

  • 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, prepares an egg to be released during ovulation and builds the lining of the womb up again after your period.
  • The second half of the cycle, the luteal phase, prepares the uterus for pregnancy and if pregnancy doesn’t occur, starts the next cycle by triggering your period.

The menstrual cycle: more than just your period

When we talk about the menstrual cycle, our periods seem to get all the air time. But the menstrual cycle is far more than that, with our periods only making up a very small part of the whole monthly process.

But thanks to cultural stigmas, taboos and limited sex education, it’s no surprise that many of us have grown up knowing very little about the ups and downs of our menstrual cycles. 

Not only is your menstrual cycle super important for your fertility, but it’s intimately linked to your overall health. It can easily be influenced by other factors such as stress, diet, weight fluctuations, exercise, sleep, illness and medications.

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.

What is the menstrual cycle?

The menstrual cycle is the bodily process that makes pregnancy possible. It involves a series of natural changes in hormone production that affect the uterus and ovaries. 

Your cycle starts on the first day of your period (the first day you bleed) and ends on the first day of your next period. It’s made up of both the ovarian cycle, which affects the ovaries and regulates ovulation, and the uterine cycle, which affects the uterus. 
Both of these cycles happen in tandem and are carefully regulated by your incredible cycling hormones—oestrogen, progesterone, luteinising hormone (LH) and follicle-stimulating hormone (FSH).

What happens during the ovarian cycle?

During the ovarian cycle, one of your ovaries will develop an egg which will be released mid-cycle, during ovulation. The ovarian cycle includes three main phases, the follicular phase, ovulation and the luteal phase. Let’s look at each phase in detail.

The follicular phase

Day 1 of your period is counted as day 1 of your menstrual cycle—it’s also when the follicular phase begins. 

For most people, this phase lasts around 10-16 days, ending mid-cycle, around ovulation. (although this can vary from cycle to cycle and person to person). Changes to the length of your follicular phase are usually the main reason why your cycle length may vary from month to month. 

During the follicular phase, a selection of immature eggs in one of your ovaries begins to mature. One egg will reach full maturation and be prepped for release during ovulation.

The follicular phase kicks off when gonadotrophin-releasing hormone (GnRH) is secreted from your brain, which promotes the release of follicle-stimulating hormone (FSH). 

FSH stimulates your follicles—little sacs in your ovaries containing immature eggs, and a few selected follicles will begin to grow and mature. As these follicles grow, they secrete oestrogen—the main female sex hormone. 

Thanks to the rise in oestrogen, generally, this time in your cycle you’ll likely be feeling your best, most confident, sexy and unstoppable. Time to schedule that date and promotion discussion. 

However, not all of the follicles make it. Only one follicle will be picked as the chosen one for that month and the others will stop maturing. This chosen one is called the dominant follicle and will be prepped for release during ovulation.


Oestrogen gradually increases during the follicular phase and peaks around the middle of the cycle. This triggers 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. 

Ovulation takes place 28-36 hours after the onset of the LH surge. The released egg will travel through the fallopian tubes, where it will wait in hopes of being fertilised by a sperm.  

Ovulation usually happens around the middle of your cycle, if you’re having regular periods. This is usually estimated to be around day 14 of your cycle, but this is based on the assumption that everyone has a 28-day cycle, which is not always the case. The date of ovulation can fall anywhere between days 11 to 16 of your cycle.

During ovulation, you are at your most fertile. Your fertile window is the 5 days leading up to ovulation and the day of ovulation itself. This is when you’ll be most likely to conceive. 

If you’re trying to conceive, or using natural birth control methods, there are a few things you can use to track ovulation

The luteal phase

The luteal phase of the menstrual cycle lasts from the day after ovulation until the day before your next period. It lasts about 14 days and usually, this is the same in each cycle. 

After you’ve ovulated, the empty follicle that released the egg will get converted into a structure called the corpus luteum. This will produce a hormone called progesterone, which is important for pregnancy, and small amounts of oestrogen. 

Progesterone thickens the lining of your womb, called the endometrium, in preparation for a fertilised egg to implant into it. The endometrium helps to support the growth and development of an 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) 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.

This is also the phase during which premenstrual syndrome (PMS) might rear its ugly head. 

What happens during the uterine cycle?

The uterine cycle happens alongside the ovarian cycle and involves all of the changes happening in the endometrium as it prepares to welcome in a fertilised egg. 

The menstrual phase 

This is when you’re menstruating or having your period. Your endometrium builds up during your cycle. But if no pregnancy occurs, falling oestrogen and progesterone just before your period will trigger the breakdown of the endometrium, because it’s no longer needed to support a pregnancy. 

Your endometrium is shed along with menstrual blood through your vagina during your period. 

The proliferative phase

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

The secretory phase

After ovulation, the secretory phase starts. Under the influence of progesterone, your endometrium will 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.

Menstrual cycle FAQs

When does the menstrual cycle start?

Your menstrual cycle kicks in around puberty and your first period is called menarche

Unless you get pregnant or have a health condition 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!

How do I know if my cycle is normal?

You may have heard that an average menstrual cycle is 28 days, but we now know that the majority of people don’t have a  28 day cycle.

An average menstrual cycle can last between 21 and 35 days and can vary from person to person and sometimes 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). 

What’s a normal period length?

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. 

Are my periods 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.  

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 get to know your cycle?

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. 

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.

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


  5. 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, 
  6. 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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