The Menstrual Cycle: KS3 Biology Guide

The menstrual cycle is the monthly sequence of hormonal changes that prepares the uterus for a possible pregnancy. Lasting approximately 28 days, it involves changes to the uterus lining, the release of an egg at ovulation, and is controlled by four key hormones: FSH, LH, oestrogen, and progesterone.

What are the four stages of the menstrual cycle?

The cycle is divided into four main phases. Twenty-eight days is an average; normal cycles range from about 21 to 35 days, and individual variation is completely normal.

Picture the system at its broadest scale first: the whole body — a person going about their daily life — is quietly running this hormonal programme in the background, coordinated by signals travelling in the bloodstream between the brain and the ovaries. Now zoom inward: the pituitary gland at the base of the brain releases chemical messengers, and the ovaries respond. At the cellular level, follicle cells are growing, dividing, and secreting hormones, and a single egg matures inside a fluid-filled sac. Every stage is exquisitely timed.

What is the role of FSH in the menstrual cycle?

FSH (Follicle-Stimulating Hormone) is produced by the pituitary gland at the base of the brain. It is released in the first half of the cycle and travels in the bloodstream to the ovaries.

FSH causes a follicle — a small fluid-filled sac containing an immature egg — to grow and mature. As the follicle develops, FSH also stimulates it to produce oestrogen. FSH levels rise at the start of the cycle, then fall after ovulation as rising oestrogen feeds back to suppress further FSH release (negative feedback).

In fertility treatments, doctors use injections of FSH to stimulate the ovaries to develop multiple follicles at once, increasing the number of eggs available for collection during IVF (in vitro fertilisation).

What is the role of oestrogen and LH?

Oestrogen is produced by the growing follicle in the ovary. It has two major effects:

1. It causes the uterus lining (endometrium) to thicken and become rich in blood vessels, preparing the uterus to receive a fertilised egg if one arrives.
2. When oestrogen reaches a sufficiently high level, it switches from inhibiting LH release to actively triggering it — a brief burst of positive feedback that generates the LH surge.

Oestrogen also inhibits FSH at low-to-moderate levels (negative feedback), preventing other follicles from developing once one is already maturing.

LH (Luteinising Hormone) is produced by the pituitary gland. Its sharp surge around days 13–14 is the signal that triggers ovulation: the follicle wall ruptures and the mature egg is released into the Fallopian tube. After ovulation, LH also causes the empty follicle to transform into a temporary gland called the corpus luteum.

What does progesterone do?

Progesterone is produced by the corpus luteum (the remnant of the ruptured follicle). Its primary role is to maintain the thick, blood-vessel-rich uterus lining throughout the second half of the cycle — keeping the uterus ready for a fertilised egg to implant. It also inhibits both FSH and LH, preventing another follicle from developing during this phase.

Two very different outcomes follow, depending on whether fertilisation occurs:

If fertilisation occurs: the developing embryo produces a hormone called hCG (human chorionic gonadotropin), which signals the corpus luteum to keep producing progesterone. The uterus lining is maintained, and pregnancy continues. hCG is the hormone detected by home pregnancy tests.

If fertilisation does not occur: the corpus luteum degenerates after about 12 days, progesterone levels fall sharply, the uterus lining can no longer be maintained and begins to break down — triggering menstruation on day 1 of the next cycle.

How do the four hormones interact throughout the cycle?

The cycle is driven by a sequence of negative and positive feedback loops between the pituitary gland and the ovaries.

The sequence runs as follows: FSH rises → follicle develops → oestrogen rises → oestrogen (at low levels) inhibits FSH (negative feedback, limiting follicle numbers) → oestrogen (at high levels) triggers LH surge (positive feedback) → ovulation → corpus luteum forms → progesterone rises → progesterone inhibits FSH and LH → if no pregnancy, corpus luteum fails → progesterone drops → cycle restarts with menstruation.

What is the menstrual cycle's link to pregnancy?

Ovulation, around day 14, is the only point in the cycle when an egg is available to be fertilised. The egg survives for approximately 12–24 hours after release. Sperm, however, can survive in the female reproductive tract for up to five days, meaning the fertile window extends from roughly days 9 to 15 (though this varies because the timing of ovulation is not fixed).

If fertilisation occurs in the Fallopian tube, the zygote (fertilised egg) travels to the uterus over five to seven days, arriving as a ball of cells called a blastocyst, and implants into the thickened endometrium. The hCG it produces maintains the corpus luteum and therefore progesterone — holding the uterus lining in place and sustaining the pregnancy. If no fertilisation occurs, the corpus luteum degenerates, progesterone falls, and menstruation begins again.

Frequently asked questions

Why does the menstrual cycle vary from person to person?

The 28-day figure is an average; normal cycles range from 21 to 35 days, and even in one individual the length can vary from month to month. The timing of the LH surge and therefore ovulation is influenced by body weight, stress, illness, exercise level, and underlying hormonal conditions. Irregular cycles are very common in the first few years after menstruation begins (menarche) and do not necessarily indicate a medical problem. Regular, predictable cycles typically establish themselves during the mid-teens as the hormonal system matures.

How do combined oral contraceptives work?

The combined pill contains synthetic oestrogen and progestogen (a synthetic form of progesterone). These maintain high levels of both hormones throughout the cycle, mimicking the post-ovulation hormonal state. High progesterone inhibits FSH (preventing follicles from maturing) and inhibits LH (preventing the LH surge and therefore ovulation). High oestrogen also contributes to FSH suppression. Additionally, progesterone thickens cervical mucus, making it harder for sperm to pass through. Because FSH is suppressed and no follicle matures, no egg is released, so fertilisation cannot occur.

What is the difference between the menstrual cycle and puberty?

Puberty is the broader, one-time process of physical and hormonal development that typically begins between ages 9 and 14, during which the reproductive system matures. The start of menstruation — called menarche — is one of the later events in puberty, typically around age 12–13 (though ages 10–16 are all within the normal range). The menstrual cycle is the recurring monthly hormonal cycle that follows menarche and continues until menopause, typically in the late 40s to early 50s. Puberty happens once; the menstrual cycle happens roughly monthly for several decades.

What causes period pain?

Period pain (dysmenorrhoea) is caused by the uterus contracting to shed its lining. During menstruation the uterus produces prostaglandins — hormone-like substances that stimulate muscle contractions. Higher concentrations of prostaglandins are associated with stronger contractions and more pain. During intense contractions the blood supply to the uterine muscle briefly decreases, producing a cramping sensation similar to a muscle cramp elsewhere in the body. Anti-inflammatory medicines such as ibuprofen work by reducing prostaglandin production, which is why they are effective for mild to moderate period pain. Severe or worsening pain may indicate an underlying condition such as endometriosis and warrants medical assessment.

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