The Menstrual Cycle Pt. 1 - The Ovarian Cycle

Menstruation is something that half of our population experiences, but is half of our population aware of what's actually happening in their bodies? MonthlyMatters’ goal is to increase menstrual health awareness and understanding in both menstruators and non-menstruators. 

The menstrual cycle is divided between the ovarian cycle and uterine cycle. The ovarian cycle structures phases based on changes in the ovaries, while the uterine cycle structures phases on the changes in the uterus. In this article, we will be covering the ovarian cycle, and part 2 of this article about the uterine cycle will be uploaded soon after. 

The Follicular Phase

The follicular phase occurs from the start of the period until ovulation happens. At the start of this phase, the lining of the uterus is thick and full of nutrients that would nourish a potential implanted egg. The pituitary gland in the brain produces a follicle-stimulating hormone (FSH), which signals the ovaries to start preparing for ovulation by supplying an egg. Several fluid-filled sacs containing eggs called follicles develop in both ovaries. While others remain small, one follicle becomes the dominant one, and grows to around 2cm in size.

When the dominant follicle is growing, it is released estrogen, increasing estrogen levels in the body, which signals the anterior pituitary gland to secrete a surge of the Luteinizing hormone (LH) in the next phase. With the rise of estrogen levels, estradiol and serotonin also usually rise, brightening the mood and producing the most sociable, energetic, and positive state of mind for the menstruator, especially towards the end of the follicular phase where hormone levels are the highest.

The average follicular phase length is 13-14 days, but anything within 10-22 days is considered medically unproblematic. It is possible for one to have a long follicular phase, while still maintaining an average cycle length of around 28 days.

Menstruation happens in the first few days of the follicular phase as part of the uterine cycle, and will be explained in part 2.

The Ovulatory Phase

The next phase is ovulation. Occurring at around the 14th day of a normal 28-day cycle, it marks when the dominant follicle rises from the surface of the ovary and ruptures to release an egg into the fallopian tube. The previously mentioned surge of the Luteinizing hormone causes the egg to be released.

Some menstruators experience a mild pain at the time of ovulation on the side where the egg is released, for reasons that are not yet well-researched. It is also normal to have ovulation spotting, where severe changes in the hormone levels during this phase causes some uterine lining to shed, releasing a few drops of blood.

Against popular misconception, the ovaries do not alternate in the role of releasing the egg. Instead, the pattern seems to be random. The most prominent reason why menstruators track their ovulation days is to plan for pregnancy. The fertile period of an egg in the fallopian tube is only 1 day, however sperm has a life span of around 5 days, meaning the average fertile window is around 6 days long.

Because the fertile window is so short, the body tries to increase the likelihood of pregnancy by peaking the level of estrogen, leading menstruators to feel a higher level of attractiveness and desirability. The rise of testosterone in this phase also increases the body’s level of sexual function and drive. 

The Luteal Phase

The last phase of the ovarian menstrual cycle is the luteal phase. This phase takes place after ovulation and before the next period starts, lasting about 14 days but ranging from 9-16 days. During this phase, the egg slowly moves through the fallopian tube, and the ruptured follicle closes to form a structure called the corpus luteum. This yellow mass of cells on the surface of the ovary releases large quantities of the progesterone hormone. The primary role of progesterone is to create a suitable environment for a possible pregnancy to occur in the uterus.

Some ways it creates this environment is by thickening the endometrium (uterine wall) with fluids and nutrients to nourish an embryo, thickening cervical mucus to stop bacteria from entering the uterus, and increasing basal body temperature (lowest resting body temperature). Estrogen levels still remain high during this phase, aiding progesterone in thickening the endometrium.

Though the progesterone levels in the body are meant to prepare menstruators for pregnancy, it also causes many of the negative side effects often associated with the luteal phase. Symptoms such as bloating, poor fluid retention, and fatigue are commonly reported.

If pregnancy does not occur, the corpus luteum breaks down and all of the hormones that have been built up throughout the menstrual cycle suddenly drop. When estrogen, progesterone, and testosterone levels are at their lowest during this time, menstruators experience physical symptoms like breast tenderness, acne, cramps, and headaches, and emotional symptoms like exhaustion, irritability, anxiety, depression, and less sexual desire. The progesterone and estrogen at the end of the luteal phase sheds the uterine lining, signalling the pituitary gland to release more FSH, starting a whole new cycle.

If pregnancy does occur, cells surrounding the developing embryo release a hormone called human chorionic gonadotropin (hCG) to retain the corpus luteum, sustaining the progesterone levels until the embryo can produce its own hormones.

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Cleveland Clinic. (2021, July 10). Corpus luteum: Development, anatomy & function. Cleveland Clinic. https://my.clevelandclinic.org/health/body/21849-corpus-luteum

Cleveland Clinic. (2022, December 9). Menstrual cycle. Cleveland Clinic. https://my.clevelandclinic.org/health/articles/10132-menstrual-cycle

Jessica E. McLaughlin. (2018). Menstrual cycle. Merck Manuals. Merck Manuals Consumer Version. https://www.merckmanuals.com/home/women-s-health-issues/biology-of-the-female-reproductive-system/menstrual-cycle

McCoy, A. (2012, November 29). How the phases of menstruation affect your mood. Everyday Health. EverydayHealth.Com. https://www.everydayhealth.com/womens-health/how-your-menstrual-cycle-affects-your-behavior.aspx

McWeeney, C. (2023, January 31). Ovulation bleeding 101. Clue. https://helloclue.com/articles/cycle-a-z/ovulation-bleeding-what-it-is-how-to-know-if-youre-experiencing-it

Ray, L. (2022, September 14). What is the menstrual cycle? Clue. Helloclue.Com. https://helloclue.com/articles/cycle-a-z/the-menstrual-cycle-more-than-just-the-period