Understanding the menstrual cycle: Your body’s natural rhythm

By Daphne Chong MD | June 18, 2025

Part 3, Podium & Parenthood: Fertility & family planning webinar series

Presented by Markham Fertility Centre in partnership with Canadian Sport Institute Ontario (CSIO).

For athletes, understanding the menstrual cycle is about more than reproductive health, it’s about performance, recovery, and long-term wellness. The hormonal fluctuations that shape each phase of the cycle can influence everything from energy levels and endurance to injury risk and bone density. Yet, this vital aspect of physiology is often misunderstood or overlooked in sport settings.

This educational resource explains how the body functions, what a “normal” cycle looks like, and why this information is especially important for athletes. With the right knowledge, women and those with cycles can better support both their health and their performance goals. This session focuses on understanding the menstrual cycle—a cornerstone of reproductive health. This knowledge empowers individuals, especially athletes, to make informed choices about their health, fertility, and future.

See Part 1, Podium & Parenthood: Fertility 101

See Part 2, Podium & Parenthood: Fertility Preservation

We will cover; 

  1. What is the menstrual cycle?
  2. Hormonal regulations of the menstrual cycle
  3. What is menarche? What is menopause?
  4. How is egg reserve defined over time?
  5. What is considered a normal menstrual cycle?
  6. Myths about hormones
  7. The menstrual cycle and athletic health
  8. Birth control and hormonal regulation
  9. Why understanding your cycle matters

1. What is the Menstrual Cycle?

The menstrual cycle is the monthly hormonal cycle a female body goes through to prepare for a potential pregnancy. It involves the interaction of multiple organs and hormones, primarily governed by the brain (hypothalamus and pituitary gland), ovaries, and uterus.

The menstrual cycle is divided into four main phases:

  1. Menstrual Phase: Shedding of the uterine lining (endometrium) when pregnancy does not occur. This results in menstruation (period).
  2. Follicular Phase: Begins on the first day of menstruation and continues until ovulation. During this time, follicles in the ovaries mature, stimulated by hormone signals.
  3. Ovulation: Occurs mid-cycle, typically around day 14 in a 28-day cycle. A mature egg is released from the ovary.
  4. Luteal Phase: The body prepares for a potential pregnancy. If the egg isn’t fertilized, hormone levels drop and the cycle begins again.

A “normal” cycle length ranges from 21 to 35 days. Menstruation (bleeding) typically lasts 3–8 days. The average woman will have approximately 465 periods over her lifetime, equating to nearly 2,300–2,500 total days of bleeding—or about 6.5 years.

2. Hormonal Regulation of the Menstrual Cycle

Hormones play a critical role in regulating the menstrual cycle. The process is coordinated through the hypothalamic-pituitary-ovarian (HPO) axis:

  • The hypothalamus, located in the brain, releases GnRH (gonadotropin-releasing hormone) in a pulsatile manner.
  • GnRH stimulates the pituitary gland, another brain structure, to secrete:
    • FSH (follicle-stimulating hormone): Stimulates growth of ovarian follicles.
    • LH (luteinizing hormone): Triggers ovulation and formation of the corpus luteum, which produces progesterone.

In response, the ovaries produce:

  • Estrogen: Dominates the first half of the cycle (follicular phase) and helps thicken the uterine lining.
  • Progesterone: Dominates the second half (luteal phase) and maintains the uterine lining for possible implantation.

If fertilization doesn’t occur, hormone levels fall, leading to shedding of the uterine lining (menstruation).

3. Defining Menarche and Menopause

  • Menarche: The first occurrence of menstruation. In Canada and most developed countries, the average age is 12.5 years, though this is trending younger, possibly due to higher nutritional intake and increased body fat.
  • Menopause: Defined as the absence of menstruation for 12 consecutive months, signaling the end of reproductive capacity. The average age in Canada is 51 years.
  • Perimenopause: The transitional period before menopause, typically lasting 4 to 8 years, during which individuals may experience irregular cycles, hot flashes, and other symptoms due to fluctuating hormone levels.

4. Egg Reserve and Fertility Over Time

A female is born with all the eggs she will ever have. The egg count decreases naturally over time:

  • At 20 weeks gestation: 6 to 7 million eggs
  • At birth: 1 to 2 million eggs
  • At puberty: 300,000 to 500,000 eggs
  • At age 37: About 25,000 eggs remain

This reduction occurs due to a process called atresia, in which unused eggs are gradually lost. By menopause, almost no viable eggs remain, highlighting the time-sensitive nature of fertility planning.

5. What is Considered a “Normal” Menstrual Cycle?

  • Cycle Length: 21 to 35 days
  • Period Duration: 3 to 8 days
  • Blood Volume: Typically 20 to 85 mL per cycle (equivalent to 4 to 6 tablespoons)

On average, an individual changes pads or tampons every 3 hours during menstruation. Needing to change more frequently, especially at night, may signal a heavy flow.

Normal symptoms:

  • Mild cramping
  • Mood fluctuations
  • Breast tenderness
  • Changes in cervical mucus

Red flags to watch for:

  • Soaking through a pad or tampon every 1–2 hours
  • Periods lasting longer than 8 days
  • Blood clots larger than 1 inch in diameter
  • Fatigue or diagnosed anemia due to blood loss

6. Hormone Myth-Busting

Common myths surrounding menstruation include:

  • “You can’t get pregnant during your period”: False. Ovulation timing and sperm lifespan can result in pregnancy even with intercourse during menstruation.
  • “Periods sync with those of other women”: Although often reported, there is no consistent scientific evidence to support this claim.
  • “Hormones should be balanced all the time”: Not true. Hormones naturally rise and fall throughout the cycle. There is no fixed “balance” to maintain.

7. The Menstrual Cycle and Athletic Health

Athletes face unique menstrual challenges:

  • Low energy availability (high output vs. low intake) can reduce GnRH release, disrupting the hypothalamus- pituitary- ovarian.
  • This leads to anovulation, irregular cycles, or amenorrhea (no periods).
  • Estrogen, critical for bone health, may fall to unsafe levels.

Without regular cycles, young athletes may fail to reach optimal bone mineral density, increasing long-term risk of osteopenia, osteoporosis, and stress fractures.

8. Birth Control and Hormonal Regulation

Hormonal contraception is categorized by content and delivery:

  • Combination methods: Include both estrogen and progestin. Found in pills, vaginal rings, and some patches.
  • Progestin-only methods: Include IUDs, injections (e.g., Depo-Provera), and mini-pills. Often long-acting.

Non-contraceptive benefits include:

  • Regulation of heavy or painful periods
  • Management of hormonal acne and hirsutism
  • Reduced risks of endometrial and ovarian cancers
  • Bone health preservation in those with menstrual dysfunction
  • Symptom relief for polycystic ovary syndrome (PCOS)

9. Why Understanding Your Cycle Matters

Knowing your cycle helps you:

  • Recognize signs of hormonal imbalance or health concerns
  • Plan for conception or contraception
  • Align training and nutrition with physical performance
  • Address symptoms early, reducing long-term health risks

Fertility and the menstrual cycle is more than a monthly inconvenience—it is a vital sign of reproductive and systemic health. By understanding the interplay of hormones, physical changes, and timing, individuals are better equipped to advocate for their wellbeing.

Markham Fertility Centre and Canadian Sport Institute Ontario are proud to support informed, proactive healthcare that empowers women and those with cycles to thrive both in life and sport.

Helpful resources

Posted in Natural Conception, Diagnostic Testing