# Understanding the Menstrual Cycle: Divisions, Phases, and Processes >[!example] 10 June 2023 #articles > >I tried to make the complex simple in this article. But realised that it was still not enough. Hence I took out a couple of thousand words and created [[The Menstrual Cycle Basics|a doubly simplified version]]. That said, I have still published both of them. Why? Because they both have value. If you want an overview, head to the [[The Menstrual Cycle Basics|smaller version]]. If you want a more in-depth look at the science, continue reading this one. Or better still, read both! %%[[a woman will have around 500 menstrual cycle's in her lifetime]]%% As women, we'll have approximately 500 menstrual cycles throughout our lives.[^1] That's a staggering 37 years of cycles which can be a challenging experience to navigate. Especially if we don't know what's going on or why. But it's incredible how much our experience can shift through knowledge. My articles on this site aim to do precisely that. And it all begins with this one. After all, many people believe that the menstrual cycle is a recurring event of 28 days. A process in which the endometrial lining of the uterus prepares for pregnancy. And if pregnancy does not occur, the uterus sheds its lining at menstruation. But this is only part of the story. The menstrual cycle is not just about periods. It is not always 28 days. And it does not just involve the uterus. %%[[oestrogen & progesterone affect the whole body]]%% It is a beautiful, complex and multifaceted process. One that permeates every aspect of our being - from our physical form to our thoughts and emotions.[^2] ## Before we begin There are a few things I want to make clear before we get into the bulk of the article. ### 1. Menstrual Cycle vs Female Reproductive Cycle Menstrual implies menstruation, our period, the building and the shedding of the endometrium. But again, the cycle that we experience is more than just bleeding. It is the combination of the uterine cycle and the ovarian cycle. And it involves the brain, ovaries and uterus. It also impacts the rest of our body, moods, thoughts, and behaviours. %%[[the menstrual cycle is more accurately termed the female reproductive cycle]]%% For this reason, we should probably refer to the menstrual cycle as the female reproductive cycle, as some researchers suggest.[^3] But I don't like that term either, as although the body's focus is procreation, it's not *our* sole focus. That said, the menstrual cycle is the term that most people use and understand, so it's the one I will stick with. I just wanted to highlight (again) that the menstrual cycle is not only about periods. ### 2. 28 Days? As mentioned above, 28 days is the default that textbooks use to explain the menstrual cycle. But women are never textbook. %%[[menstrual cycles are much more variable than the textbook definition of 28 days]]%% And menstrual cycles are *much* more variable than the textbook definition of 28 days. %%[[most women have a menstrual cycle of between 23 & 35 days long]]%% %%[[only 12.4% of women actually had a 28-day cycle]]%% Most women have a menstrual cycle of between 23 & 35 days long. And in fact, only 12.4% of women actually have a 28-day cycle.[^4] That said, for simplicity, I will use the default 28-day cycle in the rest of the article should I need to state a day. I know this isn't ideal, and I don't like doing it. But sometimes, it helps and puts things into context. ## Dividing the menstrual cycle ![[menstrual uterine ovarian cycle.excalidraw.png]] The menstrual cycle is a series of events that include the brain, ovaries and uterus. So it's complex. And when something is complex, it's normal for us to try and simplify things. The menstrual cycle is no exception. Hence there are many divisions that we can identify. ### Pre-Ovulation & Post-Ovulation In the simplest of terms we can divide the the menstrual cycle into 2 phases: 1. Pre-Ovulation 2. Post-Ovulation Pre-ovulation begins on day 1 (the first day of our bleed) and lasts until ovulation (around day 14-15). Post-ovulation starts immediately after ovulation and lasts until our next bleed. Did you notice that Ovulation is the term used in both of these phases? Know why? It's because [[Why Ovulation Is the Key to Optimal Health|ovulation is the main frickin' event]] in our menstrual cycle. And not, as we have been led to believe, our period. ### The Uterine Cycle & Ovarian Cycle We can also divide the menstrual cycle can into 2 smaller cycles: 1. The Ovarian Cycle 2. The Uterine Cycle %%[[the ovarian cycle encompasses events in the ovaries]]%% The ovarian cycle focuses on events in the ovaries that occur during and after the development of an egg. %%[[the uterine cycle is a series of changes in the endometrium of the uterus]]%% The uterine cycle is a series of changes in the endometrium, the lining of the uterus. These are the events that take place to prepare it for pregnancy. We could also refer to the uterine cycle as the *real* menstrual cycle as it involves menstruation, as we will see later. ## Understanding the menstrual cycle ![[hormones of the menstrual cycle.excalidraw.png]] Now that we have identified how the menstrual cycle can be divided, it's much easier to discuss. But again, please remember that this is a simplification. It's a way to put things in boxes, to create order. In reality, it's not as straightforward as this. ## Pre-ovulation ![[pre-ovulation.excalidraw.png]] As mentioned above, the preovulatory phase begins on day 1 (the first day of our bleed). It lasts until ovulation (around day 14-15).  Within the preovulatory phase, there are other phases to consider.  1. The follicular phase - which is part of the ovarian cycle.[^5]  2. Menstruation and the proliferative phase - which are both part of the uterine cycle. ### Ovarian cycle: the follicular phase The follicular phase is one-half of the ovarian cycle. The other is the luteal phase. As with the pre-ovulatory phase, the follicular phase runs from day 1 to ovulation. %%[[the length of the follicular phase is 10-22 days (15.7 days on average)]]%% The average length of this phase is 15.7 days and not 14 days, as stated in textbooks.[^6] That said, every woman is different. And so is her cycle length. Hence the follicular phase length can be anything from 10 days to 22 days. %%[[variance in length of the menstrual cycle is due to differences in the follicular phase (not the luteal phase which is consistent)]]%% In fact, any variance in the length of the menstrual cycle is due to differences in this follicular phase.[^7] This means that if you have a long menstrual cycle, this phase will be longer. And if you have a shorter menstrual cycle, this phase will be shorter. This is because the luteal phase is always a set length. Something which we will look at in a moment. But for now, bear in mind that it is the follicular phase that can vary in length. Whether that's from person to person or from cycle to cycle. #### The 3 stages of development %%[[follicle means a little bag]]%% The term follicular originates from the Latin word 'folliculus' meaning "a little bag".[^8] %%[[the term "follicular phase" is used to describe the process of the ovarian follicles maturing and developing oocytes]]%% The purpose of a bag is to carry things. In this case, each little bag, each follicle, carries a single egg, also known as an oocyte.[^9] The word oocyte combines the elements *oo*, meaning 'egg', and *cyte*, meaning 'of a cell'. Oocyte, therefore, translates to 'an egg mother-cell'.[^10] In a nutshell, the follicular phase is when the body selects, matures and develops an egg. It's when it prepares for ovulation. %%[[development of the dominant follicle has been described in three stages]]%% This process divides into three stages:[^11] 1. Recruitment 2. Selection 3. Dominance ##### 1. Recruitment %%[[development of the dominant follicle has been described in three stages#Recruitment]]%% The recruitment stage occurs during days 1 through 4 of the menstrual cycle. And it begins with the brain. %%[[GnRH is secreted by the hypothalamus]]%% The hypothalamus is a region of the brain which we refer to as 'The Governor'. This is because it extends its influence to almost every organ of the body. In this case, the hypothalamus secretes gonadotropin-releasing hormone, known (thankfully) as GnRH. GnRH, as the name suggests, is a hormone. A potent little chemical messenger molecule. %%[[GnRH stimulates the release of FSH and LH]]%% It sends a message to the anterior pituitary gland. This small, pea-sized gland sits below your hypothalamus at the base of your brain. The message? To release follicle-stimulating hormone (FSH).[^12] %%[[FSH initiates follicular growth]]%% %%[[FSH promotes the development of several primordial follicles into primary follicles and then into secondary follicles]] %% Let's break this down a little. Follicle. Stimulating. Hormone. So this is a *hormone* that *stimulates* the *follicles*. And in doing so, the ovaries recruit a handful of these ovarian follicles. Hence the recruitment stage. These chosen few are then encouraged to mature and develop. First into primary follicles. Then into secondary follicles. %%[[secondary follicles begin to secrete oestrogen and inhibin]]%% And once they become secondary follicles, they begin to secrete oestrogen. This explains the increase of oestrogen in our bodies during this time. ##### 2. Selection %%[[development of the dominant follicle has been described in three stages#Selection]]%% The body selects one follicle between cycle days 5 and 7 ( roughly). She is the chosen one.[^13] The Queen. The one that will go on to ovulate.[^14] ##### 3. Dominance %%[[development of the dominant follicle has been described in three stages#Dominance]]%% %%[[the one dominant secondary follicle becomes the mature follicle]]%% %%[[the mature follicle continues to increase the production of oestrogen]]%% By day 8, the chosen one, the Queen, exerts her dominance.[^15] She demonstrates this by promoting her own growth. And as a result of her larger size increases her production of oestrogen.[^16] But what about her ladies in waiting? The chosen few recruited alongside her in the initial stages? They have done everything required of them. And now they must leave the party. Literally. %%[[oestrogen signals the brain to slow down FSH production causing all other follicles to stop growing and disintegrate]]%% The huge amounts of oestrogen that the Queen produces feedback to the brain. It sends a message ordering the reduction of FSH. Hence the stimulation of the follicles comes to a halt. The ladies-in-waiting stop growing. Then via the process of apoptosis, they sacrifice themselves. A programmed cell death. As Philipp Dettmer describes, > "Apoptosis is basically a calm suicide that keeps the contents of the cells nice and tidy." ##### The development of follicles take several months %%[[the development of follicles take several months]]%% This developmental process may take several months.[^17] So a follicle may begin to develop at the beginning of a particular menstrual cycle. But it may only reach maturity and ovulate several menstrual cycles later. Many of us already know stress can cause issues with our menstrual cycle. But did you know it may not show up when we experience it? In fact, any irregularities may be due to stress from *a few months ago* when our follicles were developing. ### Uterine cycle During this preovulatory phase, the uterine cycle divides into 2: 1. Menstruation 2. Proliferative phase #### Menstruation %%[[menses comes from the latin term ‘mensis’ meaning ‘month’]]%% Ah, menstruation. The big bleed. The time that everyone is familiar with. Where the term 'menstrual cycle' originates. A phrase which has its root in the Latin word *mensis*, meaning "month".[^18] But what actually is menstruation? What's happening? And what causes it? Menstruation occurs because, right before our period, progesterone and oestrogen levels both plummet. This stimulates the release of prostaglandins. These are hormone-like compounds which tell the body to *do* something.[^19] %%[[menstruation occurs when prostoglandins cause constriction of the arterioles in the uterus resulting in endometrial ischemia]]%% The prostaglandins send instructions to the uterus's arterioles (the smallest arteries). And the arterioles obey the commands to constrict, to close up, to contract. To shut off the blood supply. Without blood, oxygen is unable to make its way to the cells. They become oxygen depleted and die. The uterus breaks down and sheds its lining, the dead cells. An endometrial sloughing.[^20] Your period. %%[[menstruation starts on day one]]%% The first day of menstruation marks the beginning of a brand-new cycle. #### Proliferative Phase %%[[with regards to the uterine cycle the preovulatory phase is also termed the proliferative phase]]%% After death comes rebirth, a phoenix rising from the ashes. In this case, the proliferation of the uterus. Proliferation, of course, means development, growth, and expansion. %%[[menstruation is an inflammatory process]]%% %%[[there is a healing process after menstruation]]%% And this makes sense. Menstruation is an inflammatory process.[^21] So after the bleed, our body does what it needs to build itself back up. It repairs and thickens the endometrium (the lining of the uterus). %%[[oestrogen prepares the uterus for pregnancy by stimulating the repair and thickening of the endometrium]]%% %%[[during the follicular phase the thickness of the endometrium doubles to about approximately 4–10 mm]]%% This happens thanks to the rising of oestrogen. Oestrogen is a stimulant. A developer. Like putting fertiliser on your lawn. It makes things grow. Replacing all that was lost during menstruation. Doubling the thickness of the endometrium. Preparing once again for the potential baby that will arrive shortly.[^22] ## Ovulation Oestrogen has been rising and rising, as we have already seen. This is thanks to the dominant follicle, the Queen. %%[[high levels of oestrogen result in positive feedback]]%% %%[[high levels of oestrogen stimulates more frequent release of GnRH from the hypothalamus]]%% When oestrogen hits a certain level, it creates positive feedback. It sends a signal to the hypothalamus, stimulating it to release GnRH more frequently. %%[[high levels of oestrogen stimulates gonadotrophs in the anterior pituitary to secrete LH]]%% %%[[GnRH promotes the release of FSH and additional LH by the anterior pituitary]]%% GnRH then promotes the release of LH from the anterior pituitary gland. LH stands for luteinising hormone. Present for only a tiny moment in the cycle, its job is to get ovulation going. To move the cycle into the next phase, the luteal phase. %%[[the LH surge occurs roughly 34 to 36 hours prior to ovulation]]%% %%[[ovulation occurs around 9-12 hours after the peak of the LH surge]]%% %%[[LH triggers ovulation]]%% This LH surge occurs roughly 34 to 36 hours before ovulation. Once LH levels peak, ovulation will occur 9-12 hours later.[^23] %%[[ovulation is the rupture of the mature follicle and the release of an egg]]%% But what actually is ovulation? In the simplest terms, it's when the little bag releases the egg. More accurately, it's the rupture of the mature follicle resulting in the release of the oocyte. The egg (which survives for 12-24 hours) enters the fallopian tube. Here it may be fertilised before moving to the uterus.[^24] The ovulatory phase is the shortest menstrual cycle phase, between 24 and 48 hours. %%[[the most common day of ovulation was Day 15]]%% Day 15 is the most common day for a woman to ovulate, although days 14 to 17 are normal (based on a 28-day cycle).[^25] But remember that the follicular phase can vary in length. So if you have a long cycle, then you will ovulate later. And if you have a short cycle, then you will ovulate earlier. ## Post-Ovulation ![[post-ovulation.excalidraw.png]] The post-ovulatory phase lasts from the day of ovulation to the day before we get our period. It includes both the luteal phase of the ovarian cycle and the secretory phase of the uterine cycle. %%[[the length of the luteal phase is 9-18 days (13.3 days on average)]]%% This is the most constant part of the female reproductive cycle. It usually lasts 9-18 days, with 13 days being the average (the reason for which I will explain in a moment). ### Ovarian Cycle - Luteal Phase %%[[the remains of the dominant follicle become the corpus luteum during the luteal phase]]%% Ovulation is where the egg leaves the follicle. But that's not the end of the story for the little bag. The luteal phase is where the follicle, or rather what's left of it, changes. It becomes a corpus luteum, a 'yellow body'. This corpus luteum is a temporary endocrine gland. And its job is to produce progesterone during the second half of the cycle. Hence if there is no ovulation, there is no corpus luteum. And if there is no corpus luteum, there is no progesterone.[^26] %%[[high levels of progesterone inhibit the secretion of GnRH and LH]]%% High levels of progesterone inhibit the secretion of GnRH and LH. Which makes sense. At this point in the cycle, the body prepares for pregnancy (even if we did not conceive). So why waste energy on other eggs. %%[[the corpus luteum produces progesterone & oestrogen, relaxin & inhibin during the luteal phase]]%% During the second week of the luteal phase, the corpus luteum also produces oestrogen. Hence we see a rise again. This is so that the body can make one last effort to prepare the uterus for pregnancy (as we'll see in a moment). #### What happens if we don't conceive? %%[[corpus luteum function begins to decline 9-11 days after ovulation]]%% Around 9-11 days after ovulation, the corpus luteum deteriorates.[^27] Its functions decline. It slows down the release of both oestrogen and progesterone. If we do not conceive, then the corpus luteum will die altogether. %%[[if we do not conceive the corpus luteum function will stop within 14 days after ovulation (and will become the corpus albicans)]]%% This death of the corpus luteum happens around 14 days after ovulation. After death, the corpus luteum becomes known as the corpus albicans, the ‘white body’.[^28] %%[[we do not know what triggers the death of the corpus luteum]]%% How this happens and what triggers it is still a mystery. Yet it does happen in every cycle that we do not conceive. The corpus luteum dies. And our hormones plummet. #### What happens if we do conceive? %%[[if we conceive the corpus luteum will persists past its normal 2-week life span]]%% But if we do conceive, what stops our body from menstruating? If the egg becomes fertilised, it continues to make its way down the fallopian tube to the uterus. The corpus luteum receives a message. Its job is to continue making progesterone and oestrogen. It’s saved. Now it needs to support the pregnancy through the initial stages. This means the corpus luteum will persist past its normal 2-week life span. And no drop in hormones means no menstruation. #### Why is the luteal phase constant? ![[variations in menstrual cycle length.excalidraw.png]] %%[[Understanding the Menstrual Cycle]]%% I mentioned earlier that any variations in the menstrual cycle length are due to the follicular phase. I also said that the luteal phase is more constant and consistent. The reason for this, I'm sure you have already worked out, is because of the corpus luteum. The 'yellow body' only survives for two weeks. So the length of the luteal phase is based on how long the corpus luteum maintains its hormone production. As I have already said, this phase tends to be pretty set and lasts 9-18 days. It may be shorter than this if there is a lack of progesterone (this may be the case during perimenopause).[^29] But it is rarely longer than this timeframe. Unless, of course, you have conceived. ### Uterine Cycle - Secretory Phase %%[[progesterone and oestrogen further prepares the endometrium for possible pregnancy]]%% %%[[the purpose of the corpus luteum is to produce progesterone to prepare the endometrium for pregnancy]]%% I mentioned above that the corpus luteum produces progesterone and oestrogen. And it's these hormones that further prepare the endometrium for a possible pregnancy. %%[[oestrogen builds the house, progesterone furnishes it]]%% To use an analogy, where oestrogen builds the house, progesterone furnishes it. So the uterus lining continues to build, mature & stabilise. %%[[peak vascularisation of the endometrium is achieved 8-9 days after ovulation]]%% At around 8 to 9 days after ovulation, the house is ready.[^30] This is when peak vascularisation of the endometrium occurs. Meaning that there is enough blood supply to take care of a baby. Talk about perfect timing. This is around the same time we expect implantation (should it occur). %%[[if fertilisation does not occur, hormones wane leading to menstruation]]%% But if a baby does not move into its home, the corpus luteum will die ( as we have seen). Oestrogen and progesterone drop. The body tells the uterine lining its time to go - menstruation . And the cycle begins again. ## Conclusion The menstrual cycle is a beautiful, complex and multifaceted process. It's not just about periods. It is not always 28 days. And it does not just involve the uterus. It is a process that permeates every aspect of our being - from our physical form to our thoughts and emotions. And seeing as they'll be part of our story for an astonishing 37 years, isn't it worth knowing what's going on? My articles on this site aim to educate about all factors of the menstrual cycle. But it all begins with this one. [^1]: Jones, R.E. and Lopez, K.H. (2013) Human Reproductive Biology. Academic Press [^2]: Elliott-Sale, K.J. _et al._ (2021) ‘Methodological Considerations for Studies in Sport and Exercise Science with Women as Participants: A Working Guide for Standards of Practice for Research on Women.’, _Sports Med_, 51(5), pp. 843–861. Available at: [LINK](https://doi.org/10.1007/s40279-021-01435-8). [^3]: Tortora, G.J. and Derrickson, B.H. (2017) Principles of Anatomy and Physiology. United States of America: Wiley Global Education. [^4]: Soumpasis, I., Grace, B. and Johnson, S. (2020) ‘Real-life insights on menstrual cycles and ovulation using big data’, Human Reproduction Open, 2020(2), p. hoaa011. Available at: [LINK](https://doi.org/10.1093/hropen/hoaa011). [^5]: So much of the literature is inconsistent in their menstrual cycle phase definitions. This is something I have [[The Flaws in Female-Based Research|written about here.]] [^6]: Schmalenberger, K.M. _et al._ (2021) ‘How to study the menstrual cycle: Practical tools and recommendations.’, Psychoneuroendocrinology, 123, p. 104895. Available at: [LINK](https://doi.org/10.1016/j.psyneuen.2020.104895). [^7]: Schmalenberger, K.M. _et al._ (2021) ‘How to study the menstrual cycle: Practical tools and recommendations.’, Psychoneuroendocrinology, 123, p. 104895. Available at: [LINK](https://doi.org/10.1016/j.psyneuen.2020.104895). [^8]: [follicle | Etymology, origin and meaning of follicle by etymonline](https://www.etymonline.com/word/follicle#etymonline_v_11742) [^9]: See: [[Published/Blog/How Our Reproductive Potential is Determined Before Birth]] [^10]: I think it's interesting that the Welsh word for egg is 'wy' which is pronounced *ooee*. [^11]: Reed, B.G. and Carr, B.R. (2018) The Normal Menstrual Cycle and the Control of Ovulation, _Endotext [Internet]_. MDText.com, Inc. Available at: [LINK](https://www.ncbi.nlm.nih.gov/books/NBK279054/) [^12]: It also releases luteinizing hormone, which we will look at shortly. [^13]: %%[[nonidentical twins or triplets result when two or three secondary follicles become co-dominant and later are ovulated and fertilised at about the same time]]%% NON-identical twins or triplets result when two or three follicles become co-dominant. They *all* become Queens. And later ovulate and become fertilised at about the same time. [^14]: Anti-Müllerian hormone (AMH) is believed to play a role in the selection of the dominant follicle. [^15]: Linda J. Heffner, Danny J. Schust (2014) _The Reproductive System at a Glance_. John Wiley & Sons. [^16]: %%[[high amounts of oestrogen are produced when the dominant follicle is >15mm in diameter on ultrasound]]%%The dominant follicle produces high amounts of oestrogen. That is, once she is larger than 15mm in diameter on an ultrasound.(Reed and Carr, 2018) [^17]: Tortora, G.J. and Derrickson, B.H. (2017) Principles of Anatomy and Physiology. United States of America: Wiley Global Education. [^18]: Jones, R.E. and Lopez, K.H. (2013) Human Reproductive Biology. Academic Press. [^19]: Tortora, G.J. and Derrickson, B.H. (2017) Principles of Anatomy and Physiology. United States of America: Wiley Global Education. [^20]: %%[[menstrual flow is not just blood]]%% It’s also worth noting that the menstrual flow from the uterus isn’t just blood. It also includes tissue fluid, mucus and epithelial cells. [^21]: Liu, H.-L., Chen, D.-D., Long, C.-C., Lu, Y.-X., Qu, X.-Y., Cheng, Z.-P., 2018. Appearance of Inflammation in Peripheral Blood during Menstrual Cycles in Women of Childbearing Age. Reproductive and Developmental Medicine 2, 46–51. [LINK](https://doi.org/10.4103/2096-2924.232878) [^22]: Again, whether we want to have a baby or not, this is biology. The body is doing her job. [^23]: Reed, B.G. and Carr, B.R. (2018) The Normal Menstrual Cycle and the Control of Ovulation, _Endotext [Internet]_. MDText.com, Inc. Available at: [LINK](https://www.ncbi.nlm.nih.gov/books/NBK279054/) [^24]: Now remember that I said that *nonidentical* twins (or triplets) result when another secondary follicle becomes co-dominant? Well within this window a 2nd egg may be released and this co-dominant follicle also survives for 12-24 hours. Which means that If both are fertilised this will result in *fraternal* twins. [^25]: Soumpasis, I., Grace, B. and Johnson, S. (2020) ‘Real-life insights on menstrual cycles and ovulation using big data’, Human Reproduction Open, 2020(2), p. hoaa011. Available at: [LINK](https://doi.org/10.1093/hropen/hoaa011). [^26]: Now can you understand why ovulation is so vital? See: [[Why Ovulation Is the Key to Optimal Health]] [^27]: Reed, B.G. and Carr, B.R. (2018) The Normal Menstrual Cycle and the Control of Ovulation, _Endotext [Internet]_. MDText.com, Inc. Available at: [LINK](https://www.ncbi.nlm.nih.gov/books/NBK279054/) [^28]: Oliver, R. and Pillarisetty, L.S. (2022) ‘Anatomy, Abdomen and Pelvis, Ovary Corpus Luteum’, in _StatPearls_. Treasure Island (FL): StatPearls Publishing. Available at: [LINK](http://www.ncbi.nlm.nih.gov/books/NBK539704/) (Accessed: 13 March 2023). [^29]: %%[[↓ FSH levels during the follicular phase can lead to a shortened luteal phase and the development of a smaller corpus luteum]]%% A short luteal phase may also result from low FSH levels during the follicular phase. This can lead to the development of a smaller corpus luteum. [^30]: Reed, B.G. and Carr, B.R. (2018) The Normal Menstrual Cycle and the Control of Ovulation, _Endotext [Internet]_. MDText.com, Inc. Available at: [LINK](https://www.ncbi.nlm.nih.gov/books/NBK279054/)