Women's Health Series // Getting to Know Your Cycle

Now that we've covered most of the hormones that are important to female health and reproduction on the blog, I thought that I would write just one more post to lay the foundation for more complex and nuanced areas of conversation that I plan on sharing in future. These will hopefully include posts that will aim to answer the many questions that I've received about topics like PCOS, how nutrition impacts fertility, pregnancy, and the intersection of establishing a healthier relationship with food and one's body in the midst of our (often) complicated hormones. Today we will be looking at the female menstrual cycle, and how it ties in with the hormonal changes that we experience from month to month.


Introduction

The menstrual cycle refers to the regular, natural physiological changes that occur in the female reproductive system (specifically the lining of the uterus and ovaries) from the beginning of one menstrual period to the beginning of the next. It can be described in one of two ways - by the ovarian cycle or the uterine cycle. Women's cycle lengths are highly variable, but most commonly last between 23-35 days [1]. The process is highly controlled by endocrine, autocrine, and paracrine factors in the body that regulate each and every important step that forms part of each cycle [1].

The diagram below is a great ‘blueprint’ detailing all of the changes that take place during each menstrual cycle, including hormonal changes, ovarian follicle development, ovulation, and remodelling of the endometrium. We will go through each of these in more detail in this post, but feel free to keep referring back to this picture if at any point you get confused or just want to make sure you're on the right track when it comes to understanding how things all work together.

 
monthly_cycle_phases.png
 

hormonal changes that take place

during a menstrual cycle

I covered each of the hormones discussed below in my last blogpost in the Women's Health Series, so be sure to head to that post if you want more detailed information. To start things off, I thought it might be a good idea to briefly summarise the hormonal changes that take place throughout a menstrual cycle, as the changes in these hormones are what affect both the uterine and ovarian cycles (which will be discussed below).

  • GnRH is produced and released by the hypothalamus in response to low circulating oestrogen levels.

  • GnRH acts on the anterior pituitary gland to trigger the production and secretion of FSH and LH [2].

  • FSH and LH act on the ovaries to stimulate the maturation of ovarian follicles in preparation for ovulation.

  • As ovarian follicles develop, oestrogen levels increase to ensure that the endometrial lining thickens and prepares for implantation. Increasing oestrogen levels also ensure that FSH and LH levels do not continue to increase.

  • Oestrogen increases until a critical level is reached, which triggers the pulses in GnRH that result in a surge in LH that is responsible for ovulation [2].

  • After ovulation, a mature egg travels down the fallopian tube to the uterus, and what remains of the mature follicle becomes a corpus luteum. During this time FSH and LH levels decline, progesterone levels increase, and oestrogen levels remain elevated [2].

  • If an egg is not fertilised, the corpus luteum begins to break down, levels of progesterone and oestrogen decrease, and menstruation occurs [2].

  • If an egg is fertilised, progesterone and oestrogen levels remain elevated and the body starts to produce human Chorionic Gonadotrophin (hCG).


regulation of a normal menstrual cycle

The main ‘control system’ involved in the regulation of a woman's cycle is known as the Hypothalamic-Pituitary-Ovarian (HPO) axis. The regulation of these interactions relies upon complex negative and positive feedback loops between the hypothalamus, pituitary gland, and ovaries. The following diagram is useful for better understanding exactly how hormonal regulation in each phase of the menstrual cycle works (particularly if you are a visual learner).


The ovarian cycle

The ovarian cycle refers to the series of changes that take place in the ovaries during a menstrual cycle as follicles mature, an ovum is released, and the corpus luteum develops. It is responsible for the preparation of endocrine tissues and release of mature eggs [5]. This cycle is made up of three phases, namely the:

  • Follicular phase - The first phase of the menstrual cycle that involves the maturation of ovarian follicles, and ends with ovulation. Research has found that the most variation in cycle length between women seems to occur during this phase [2,3]

  • Ovulation - The point in a menstrual cycle where hormone changes trigger the release of a mature egg (ovum) from the ovary.

  • Luteal phase - The final phase of the menstrual cycle that begins with the formation of the corpus luteum, and ends in either pregnancy or luteolysis and menstruation. This phase tends to be more consistent between individuals and generally lasts between 12-16 days [2,3].

Let's look at each of these phases in a bit more detail:

  1. Follicular phase

During the first part of the ovarian cycle, ovarian follicles mature in preparation for the release of an egg [6]. This phase overlaps with the proliferative phase of the uterine cycle [6]. During the first few days of a cycle, a rise in FSH and LH stimulates the maturation of a few primary ovarian follicles. Each of the developing follicles compete with each other for dominance and - in most cases - only one grow into a mature follicle [6]. The follicle that reaches maturity - known as a tertiary or Graafian follicle - contains the ovum (egg) that will be released [7].

2. Ovulation

Ovulation is the point at which a mature ovarian follicle ruptures to release an egg, which travels down the fallopian tube and eventually reaches the uterus [5]. As ovarian follicles mature during the follicular phase, oestrogen levels steadily increase until a critical threshold is reached. At this point, oestrogen switches from suppressing the release of excess LH to stimulating a LH surge, which causes the mature follicle to release an egg [5].

3. Luteal phase

This is the final phase of the ovarian cycle. It overlaps with the secretory phase of the uterine cycle [6]. After ovulation, FSH and LH cause the remains of the ovarian follicle to become a corpus luteum, which secretes progesterone and stimulates the production of oestrogen. Increasing levels of these hormones suppress the production of FSH and LH, which prevents the development of any further follicles and eggs [7]. If a released egg is not fertilised after a few days, the corpus luteum degenerates, oestrogen and progesterone levels decrease, the hypothalamus will once again be stimulated to secrete GnRH, and the cycle begins again [5].

 

A summary of all the events which take place during the ovarian cycle.

 

The Uterine cycle

The uterine cycle refers to the series of changes that take place in the endometrial lining of the uterus. This cycle is made up of three phases, namely:

  • Menstruation

  • The proliferative phase

  • The secretory phase

Let's look at each of these phases in a bit more detail:

  1. Menstruation

Menstruation signals the start of a the uterine cycle. It is the most visible phase of the menstrual cycle, as it is the phase during which the endometrial lining sheds. Menstrual bleeding will usually serve as an indication that pregnancy has not occurred - if an egg is not fertilised and implantation does not occur, levels of progesterone and oestrogen decrease, and the uterine lining starts to break down. ‘Normal’ menstruation (known as eumenorrhoea) usually lasts for anywhere between 2-7 days at a time, and involves the loss of 10-80 ml blood.

2. Proliferative phase

This phase of the uterine cycle corresponds with the follicular phase of the ovarian cycle. It is during this phase where increasing levels of oestrogen cause the endometrial lining to thicken in preparation for pregnancy, if an egg ends up being fertilised [7]. During this phase oestrogen is responsible for initiating the formation of new layers of endometrium in the uterus, as well as crypts in the cervix to produce fertile cervical mucus.

3. Secretory phase

This is the final phase of the uterine cycle. It corresponds with the luteal phase of the ovarian cycle. During this phase, the corpus luteum produces progesterone, which plays an important role in preparing the endometrial lining for implantation of a fertilised egg by increasing blood flow and uterine secretions, as well as reducing contractions by uterine smooth muscle.


A summary of the ovarian and uterine cycles

Both the ovarian cycle and uterine cycle take place at the same time. The following videos are a great way to better understand exactly how the changes described in this post take place over the course of a menstrual cycle.

The events that take place during the follicular phase (menses / proliferative phase) of the menstrual cycle.

The events that take place during the luteal phase (secretory phase) of the menstrual cycle.


great resources to help you

better understand your cycle

  1. Bright Girl Health

  2. Clue period tracker & calendar app

  3. The Gynae Geek (Dr Anita Mitra) - looking forward to reading it when it is released in the UK on 7 March 2019 (this is an Amazon affiliate link, which means if you click on it and decide to purchase the book, I will make a small amount of money at no extra cost to you)


References

[1] Mihma M, Gangooly S, Muttukrishna S. The normal menstrual cycle in women. Anim Reprod Sci. 2011;124:229-36.

[2] Golden NH, Carlson JL. The Pathophysiology of Amenorrhea in the Adolescent. Ann N Y Acad Sci. 2008;1135(1):163-78.

[3] Clearblue. Menstrual cycles and ovulation [Internet]. 2018 Nov 7 [cited 2018 Dec 31]. Available from: https://uk.clearblue.com/how-to-get-pregnant/menstrual-cycles-and-ovulation.

[4] Macsali F, Svanes C, Sothern RB, Benediktsdottir B, Bjørge L, Dratva J, Franklin KA, Holm M, Janson C, Johannessen A, Lindberg E, Omenaas ER, Schlünssen V, Zemp E, Real FG. Menstrual Cycle and Respiratory Symptoms in a General Nordic-Baltic Population. Am J Respir Crit Care Med. 2013 Feb;187(4):366-73. Available from: https://www.ncbi.nlm.nih.gov/pubmed/23204251

[5] OpenStax, Biology. OpenStax CNX. 7 Nov 2018. Available from: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@11.6.

[6] Silverthorn DU. Human Physiology: An Integrated Approach. 6th ed. Glenview, IL: Pearson Education; 2013. pp. 850-90.

[7] Losos JB, Raven PH, Johnson GB, Singer SR. Biology. New York: McGraw Hill; 2002. pp. 1207-09.