Menstrual Cycle and Its Hormonal Regulation Explained

The menstrual cycle is the monthly series of changes in the ovaries and uterus that prepares a woman's body for possible pregnancy. A typical cycle lasts about 28 days, though 21 to 35 days is normal. Day 1 is counted from the first day of menstrual bleeding.

Hormonal regulation of this cycle matters because it controls ovulation, fertility and the health of the uterine lining. Understanding it explains contraceptive pills, fertility tracking and disorders such as irregular periods.

Key Concepts

• Menstrual phase (days 1-5): shedding of the endometrium as menstrual flow when no pregnancy occurs.
• Follicular or proliferative phase (days 6-14): a follicle grows in the ovary while the endometrium rebuilds.
• Ovulation (about day 14): release of the secondary oocyte, triggered by a surge of LH.
• Luteal or secretory phase (days 15-28): the corpus luteum secretes progesterone, making the endometrium thick and ready for implantation.
• FSH and LH: pituitary hormones; FSH grows the follicle, LH triggers ovulation and corpus luteum formation.
• Oestrogen: secreted by the growing follicle; rebuilds the endometrium and, at high levels, triggers the LH surge.
• Progesterone: secreted by the corpus luteum; maintains the endometrium.

Formulas

There are no mathematical formulas for the menstrual cycle, but one practical rule is used to estimate ovulation day: Ovulation day = cycle length in days minus 14, because the luteal phase is fairly constant at about 14 days. For a 28-day cycle: 28 - 14 = day 14. For a 32-day cycle: 32 - 14 = day 18. The fertile window is roughly the 5 days before ovulation plus the day of ovulation itself, since sperm survive up to about 5 days and the egg about 24 hours. This rule is an estimate, not a guarantee, and varies between individuals.

Worked Example

A woman has a regular 30-day cycle starting on 1 March (day 1). Estimated ovulation day = 30 - 14 = day 16, which falls on 16 March. Her fertile window is about 5 days before plus ovulation day: 11 March to 16 March. If no fertilisation occurs, the corpus luteum degenerates around day 26 to 28, progesterone falls, and the next period begins around 31 March, completing the 30-day cycle.

Common Misconceptions

• "Every woman has a 28-day cycle." Wrong; 21 to 35 days is normal, and cycles vary between women and across life.
• "Ovulation always happens on day 14." Only in a 28-day cycle. Ovulation is about 14 days before the next period, so it shifts with cycle length.
• "Bleeding means the egg is being released." Bleeding is the shedding of the endometrium at the start of the cycle; ovulation occurs about two weeks later.

Real-World Applications

Combined contraceptive pills work by supplying oestrogen and progesterone that suppress FSH and LH, preventing ovulation. Fertility apps and ovulation test kits (which detect the LH surge in urine) use this hormonal pattern for family planning. Doctors measure hormone levels to diagnose conditions like polycystic ovary syndrome (PCOS).

The menstrual cycle runs through menstrual, follicular, ovulatory and luteal phases, coordinated by FSH, LH, oestrogen and progesterone in a precise feedback loop repeated roughly every 28 days.