What Is the Menstrual Cycle?

Introduction

Every month, a remarkable choreography of hormones orchestrates the menstrual cycle — a biological system so finely tuned that its regularity is now considered a vital sign of overall health. Yet most people receive only the barest education about how it actually works. Understanding your cycle puts you in control: you can better predict ovulation, explain premenstrual symptoms, identify when something is wrong, and make more informed conversations with your healthcare provider.

The menstrual cycle is the monthly series of hormonal and physiological changes that prepare the female reproductive system for potential pregnancy. It is controlled by the hypothalamic-pituitary-ovarian (HPO) axis — a communication network between the brain and the ovaries. According to FIGO (the International Federation of Gynecology and Obstetrics

The Four Phases of the Menstrual Cycle

1. Menstruation (Days 1–5)

The cycle begins with menstruation. If the egg released in the previous cycle was not fertilised, the corpus luteum degenerates, causing progesterone and estrogen levels to fall sharply. This hormonal withdrawal triggers the uterine lining (endometrium) to shed — producing the menstrual flow. Bleeding typically lasts 3–7 days. Simultaneously, the pituitary gland starts releasing Follicle-Stimulating Hormone (FSH) to kick-start the next cycle.

2. Follicular Phase (Days 1–13)

Overlapping with menstruation, the follicular phase begins on Day 1 and extends until ovulation. FSH stimulates the ovary to develop a group of follicles — small fluid-filled sacs each containing an immature egg. Usually, one follicle becomes dominant and grows fastest, secreting increasing amounts of estradiol (the primary form of estrogen). This estrogen causes the endometrium to thicken and proliferate, preparing a lush lining for potential implantation. Rising estradiol also feeds back positively to the brain, eventually triggering the LH surge.

3. Ovulation (~Day 14)

When estradiol reaches a critical threshold, it triggers a sudden surge in Luteinising Hormone (LH) from the pituitary. This LH surge (which peaks roughly 24–36 hours before ovulation) causes the dominant follicle to rupture and release a mature egg into the fallopian tube. This is ovulation — the moment of maximum fertility. The egg survives 12–24 hours; sperm can survive up to 5 days in the reproductive tract, so the fertile window spans roughly Days 10–16 in a typical 28-day cycle.

4. Luteal Phase (Days 15–28)

After ovulation, the ruptured follicle transforms into the corpus luteum — a temporary endocrine gland that secretes progesterone (and some estrogen). Progesterone converts the endometrium from a proliferative to a secretory state, rich in nutrients and blood vessels. It also raises basal body temperature by about 0.2–0.5 °C. If fertilisation does not occur, the corpus luteum degenerates around Day 24–26, progesterone plummets, and the cycle begins anew with menstruation.

The Hormones in Brief

Four key hormones drive the cycle:

• FSH (Follicle-Stimulating Hormone) — stimulates follicle growth in the early follicular phase.
• Estradiol (Estrogen) — thickens the endometrium; triggers the LH surge at high concentrations.
• LH (Luteinising Hormone) — surges mid-cycle to trigger ovulation; supports the corpus luteum.
• Progesterone — prepares and maintains the endometrium in the luteal phase; prevents further ovulation.

Inhibin B (from developing follicles) suppresses FSH to prevent multiple ovulations. All four work in precise feedback loops: generally negative feedback (hormones suppress their own triggers

Why the Menstrual Cycle Matters Beyond Reproduction

Estrogen has wide-ranging effects on bones, cardiovascular health, brain function, and metabolism. The cyclic rise and fall of hormones influences mood (many women notice improved energy and focus in the follicular phase

The American College of Obstetricians and Gynecologists (ACOG) and the American Academy of Pediatrics both recommend that clinicians evaluate the menstrual pattern as a fifth vital sign, alongside temperature, pulse, blood pressure, and respiratory rate.

What Is "Normal"?

Normal ranges (per FIGO 2011):

• Cycle length: 24–38 days
• Cycle-to-cycle variation: ≤7–9 days (depending on age)
• Duration of flow: 4.5–8 days
• Volume of blood loss: 5–80 mL per cycle

Short cycles (<24 days) may indicate shortened follicular or luteal phases. Long cycles (>38 days) may signal ovulatory dysfunction, as seen in PCOS or hypothalamic amenorrhoea. Heavy bleeding (>80 mL) warrants investigation for fibroids, polyps, coagulation disorders, or hormonal irregularities.

Common Misconceptions

Myth: All women have 28-day cycles. Reality: Cycles of 24–38 days are all normal; only ~13% of women have exactly 28-day cycles. Myth: Ovulation always happens on Day 14. Reality: Ovulation timing varies with cycle length. Myth: Light periods mean low fertility. Reality: Period volume does not directly reflect fertility.

References: FIGO Menstrual Disorders Working Group 2011; StatPearls — Physiology, Menstrual Cycle 2024; ACOG Committee Opinion on Menstruation in Girls as a Vital Sign.