# The Menstrual Cycle Basics: Simplifying the Complex >[!example] 10 June 2023 #articles > The menstrual cycle is complex. And when something is complicated, it's normal for us to try and simplify things. The menstrual cycle is no exception. This is my attempt to make the complex understandable. And hopefully more enjoyable to read. This is part of a [[Understanding the Menstrual Cycle|longer article]] I have written. In case you want to learn more! ![[menstrual uterine ovarian cycle.excalidraw.png]] %%[[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. 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.[^2] 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.[^3] ## Pre-ovulation ![[pre-ovulation.excalidraw.png]] %%[[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.[^4] But it can be anything from 10 days to 22 days.[^5] %%[[variance in length of the menstrual cycle is due to differences in the follicular phase (not the luteal phase which is consistent)]]%% ### What's happening in the ovaries? %%[[follicle means a little bag]]%% This is also known as the follicular phase. The term follicular originates from the Latin word 'folliculus' meaning "a little bag".[^6] %%[[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.[^7] In a nutshell, the follicular phase is when the body selects, matures and develops an egg. It's when it prepares for ovulation. And it begins with the brain when it releases follicle-stimulating hormone (FSH). %%[[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. %%[[secondary follicles begin to secrete oestrogen and inhibin]]%% These chosen few are then encouraged to mature and develop. Once they do, they begin to secrete oestrogen.[^8] %%[[development of the dominant follicle has been described in three stages#Selection]]%% Between days 5 and 7 the body selects one follicle.[^9] The Queen. The one that will go on to ovulate.[^10] %%[[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 Queen, exerts her dominance.[^11] She demonstrates this by promoting her own growth. But what about her ladies in waiting? The chosen few recruited alongside her in the initial stages? %%[[oestrogen signals the brain to slow down FSH production causing all other follicles to stop growing and disintegrate]]%% They have done everything required of them. And now they must leave the party. Literally. Via the process of apoptosis, they sacrifice themselves. As Philipp Dettmer describes, > "Apoptosis is basically a calm suicide that keeps the contents of the cells nice and tidy." ### What's happening in the uterus? %%[[menses comes from the latin term ‘mensis’ meaning ‘month’]]%% Firstly we have menstruation. The big bleed. The time that everyone is familiar with. A phrase which has its root in the Latin word *mensis*, meaning "month".[^12] Menstruation occurs because, right before our period, progesterone and oestrogen levels both plummet. %%[[menstruation occurs when prostoglandins cause constriction of the arterioles in the uterus resulting in endometrial ischemia]]%% This stimulates the release of prostaglandins. These are hormone-like compounds which tell the body to *do* something.[^13] In this instance, the prostaglandins shut off the blood supply. Without blood, the cells become oxygen depleted and die. The uterus breaks down and sheds its lining, the dead cells. Your period. %%[[with regards to the uterine cycle the preovulatory phase is also termed the proliferative phase]]%% Then after death comes rebirth. The proliferation of the uterus. The body repairs and thickens the endometrium (the lining of the uterus). Doing what it needs to build itself back up. %%[[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. Like putting fertiliser on your lawn, oestrogen makes things grow. Doubling the thickness of the endometrium. Preparing once again for the potential baby that will arrive shortly.[^14] ## Ovulation %%[[high levels of oestrogen result in positive feedback]]%% %%[[high levels of oestrogen stimulates more frequent release of GnRH from the hypothalamus]]%% %%[[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]]%% When oestrogen hits a certain level, it sends a message to the brain to release of luteinising hormone (LH). Present for only a tiny moment in the cycle, its job is to get ovulation going. %%[[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).[^15] %%[[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.[^16] ## Post-Ovulation ![[post-ovulation.excalidraw.png]] %%[[the length of the luteal phase is 9-18 days (13.3 days on average)]]%% ### What's happening in the ovaries? %%[[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. Welcome to the luteal phase. This 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 (and oestrogen) during the second half of the cycle (which is needed in the uterus, below).[^17] %%[[corpus luteum function begins to decline 9-11 days after ovulation]]%% %%[[if we do not conceive the corpus luteum function will stop within 14 days after ovulation (and will become the corpus albicans)]]%% But around 13 days after ovulation, the corpus luteum dies.[^18] And our hormones plummet. This is why it is the most constant part of the female reproductive cycle.[^19] ### What's happening in the uterus? %%[[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.[^20] This is when there is enough blood supply to take care of a baby.[^21] %%[[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.[^22] Oestrogen and progesterone drop. The body tells the uterine lining its time to go - menstruation . And the cycle begins again. %%[[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 will persist past its normal 2-week life span. Now it needs to support the pregnancy through the initial stages. It will continue making progesterone and oestrogen. And no drop in hormones means no menstruation. ## 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]: %%[[Human Reproductive Biology by Richard E. Jones & Kristin H. Lopez]]%% Jones, R.E. and Lopez, K.H. (2013) Human Reproductive Biology. Academic Press [^2]: 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.(Soumpasis, Grace and Johnson, 2020) [^3]: %%[[(Elliott-Sale et al., 2021)]]%% 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). [^4]: %%[[(Schmalenberger et al., 2021)]]%% 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). [^5]: In fact, any variance in the length of the menstrual cycle is due to differences in this phase.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.(Schmalenberger et al., 2021) [^6]: [follicle | Etymology, origin and meaning of follicle by etymonline](https://www.etymonline.com/word/follicle#etymonline_v_11742) [^7]: The word oocyte combines the elements *oo*, meaning 'egg', and *cyte*, meaning 'of a cell'. Oocyte, therefore, translates to 'an egg mother-cell'. I also think it's interesting that the Welsh word for egg is 'wy' which is pronounced *ooee*. [^8]: This explains the increase of oestrogen in our bodies during this time. [^9]: %%[[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. [^10]: Anti-Müllerian hormone (AMH) is believed to play a role in the selection of the dominant follicle. [^11]: %%[[The Reproductive System at a Glance by Linda J. Heffner & Danny J. Schust]]%% Linda J. Heffner, Danny J. Schust (2014) _The Reproductive System at a Glance_. John Wiley & Sons. [^12]: %%[[Human Reproductive Biology by Richard E. Jones & Kristin H. Lopez]]%% Jones, R.E. and Lopez, K.H. (2013) Human Reproductive Biology. Academic Press. [^13]: %%[[Tortora's Principles of Anatomy and Physiology]]%% Tortora, G.J. and Derrickson, B.H. (2017) Principles of Anatomy and Physiology. United States of America: Wiley Global Education. [^14]: Again, whether we want to have a baby or not, this is biology. The body is doing her job. [^15]: 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. %%[[(Soumpasis, Grace and Johnson, 2020)]]%%Soumpasis, I., Grace, B. and Johnson, S. (2020) ‘Real-life insights on menstrual cycles and ovulation using big data’, Human Reproduction Open, 2020(2), Available at: [LINK](https://doi.org/10.1093/hropen/hoaa011). [^16]: 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. [^17]: So if there is no ovulation, there is no corpus luteum. And if there is no corpus luteum, there is no progesterone. Now can you understand why ovulation is so vital? See: [[Why Ovulation Is the Key to Optimal Health]] [^18]: %%[[(Reed and Carr, 2018)]]%% 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/) [^19]: It usually lasts 9-18 days, with 13 days being the average. [^20]: %%[[(Reed and Carr, 2018)]]%% 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/) [^21]: Talk about perfect timing. This is around the same time we expect implantation (should it occur). [^22]: 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.