Welcome to The Menstrual Cycle!
Today I will walk through all of the stages of the menstrual cycle and how the brain, ovaries and uterus work together. If you haven’t already, I recommend watching my first videos to learn some of the background information on follicle growth and hormones before enjoying my video above.
After watching/reading you will learn about:
Whoa new section alert! ✨Repro Relevance✨- here I talk about how this week’s topic might impact you or someone you know
let’s jump in
What is the menstrual cycle?
The menstrual cycle is a term describing the sequence of events that occurs in the body as it prepares for the possibility of pregnancy each month. It begins with the first day of the period and ends on the first day of the following period. There are 4 main parts to the cycle: menstruation, the follicular phase, ovulation and the luteal phase. On average, cycles last 28 days but range from person to person and can be anywhere between 21 to 35 days. Similarly, the time in the menstrual phase - or when bleeding occurs - also varies but is typically between 3-7 days.
Coordination between the brain, ovaries and uterus
The menstrual cycle is a coordinated sequence of events happening between the brain, ovaries and uterus. In the diagram I will be showing there are sections that describe what is happening in the brain with the brain-derived hormones FSH and LH; what is happening in the ovary both with the stages of folliculogenesis (ovarian cycle) and with the ovary-derived hormones estrogen and progesterone; and finally what is happening in the uterus (uterine cycle). This is all shown across an average 28 day menstrual cycle. In Folliculogenesis Part 1, 2 and 3 I describe the stages of the ovarian cycle (folliculogenesis) and the hormones produced in detail if you need more context. What I haven’t talked about yet is how the uterus plays a role. As mentioned, the menstrual cycle prepares the body for potential pregnancy. This involves dynamic changes in the uterine lining - the endometrium - which is where an embryo implants. These changes are known as the uterine cycle.
Refer to the diagram below throughout the next sections:
Phase 1 Menstruation
Day 0 of the menstrual cycle starts at the onset of menstruation - or bleeding. Working from top to bottom of the schematic we can see that in the brain, FSH levels are slowly starting to rise. Follicles in the ovary are beginning to grow and produce a little bit of estrogen. In the uterus, the endometrial lining is shed.
Phase 2 Follicular Phase
After menstruation, we are in the follicular phase where FSH levels remain steady triggering follicles in the ovary to grow and produce lots of estrogen. There is a steep increase in estrogen production during this time. The uterus is in its proliferative phase where the endometrial lining is thickening in preparation for a potential embryo to implant.
Phase 3 Ovulation
The peak in estrogen levels primarily from the dominant follicle in the ovary triggers a surge of LH and FSH in the brain which in turn leads to ovulation of the egg into the fallopian tube. The uterus continues to maintain endometrial thickness.
Phase 4 Luteal Phase
After ovulation, LH and FSH levels plumet and the remnant follicle in the ovary become a corpus luteum, producing progesterone. Progesterone levels trigger the uterus to enter the secretory phase where the endometrial lining maintains its thickness and produces secretions that would nourish an implanting embryo. It also develops special spiral arteries which are important to deliver nutrients to the placenta if pregnancy occurs.
And back to Phase 1 Menstruation
If no implantation occurs, there is a drop in progesterone. The spiral arteries constrict and cut off blood supply to the top layer of cells in the endometrium leading to shedding which brings us back to the menstrual phase and the cycle begins again!
✨Repro Relevance✨
So why might this matter to you or someone you know?
Period cramps
Cramping occurs due to hormone-like chemicals called prostaglandins. Prostaglandins are important for a number of things in the body including inflammation, muscle contractions and even ovulation. Their levels rise in the endometrium prior to menstruation. These chemicals cause blood vessels to constrict and also cause the uterus to contract which helps shed the uterine lining but are also the reason from the cramping sensation.
Connection between hormones and mental health
Hormone fluctuations during the menstrual cycle have been shown to influence neurotransmitters in the brain. Changing levels of estrogen and progesterone affect both serotonin and dopamine synthesis and receptor activity. This can lead to anything from mild mood changes to the more severe premenstrual dysphoric disorder (PMDD).
Next time…
Join me next time to learn about the importance of hormones for whole body health.
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References
Schematics and illustrations created with BioRender.com and Canva
Information on pain with periods: https://assets.cureus.com/uploads/review_article/pdf/373082/20250720-668375-vi423u.pdf
https://my.clevelandclinic.org/health/diseases/4148-dysmenorrhea
Connection between the menstrual cycle and neurotransmitters
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