The Menstrual Cycle

The four phases

• Menstruation (about day 1–5): the thickened uterus lining breaks down and is shed as the lining was not needed, because no fertilised egg implanted.
• Follicular phase: follicle-stimulating hormone (FSH) from the pituitary gland stimulates a follicle in the ovary to develop; the developing follicle releases oestrogen, which repairs and thickens the uterus lining.
• Ovulation (around day 14): a surge in luteinising hormone (LH) triggers the release of a mature egg from the ovary.
• Luteal phase: the ruptured follicle becomes the corpus luteum, which secretes progesterone to maintain the thickened uterus lining. If the egg is not fertilised, the corpus luteum breaks down, hormone levels fall and menstruation begins again.

The hormones involved

If fertilisation occurs

If the egg is fertilised, the developing embryo (and later the placenta) keeps producing hormones that maintain the corpus luteum and the uterus lining, so menstruation does not occur and the lining continues to support the pregnancy.

Hormonal feedback control

The four hormones regulate each other through feedback loops. Rising oestrogen from the developing follicle eventually triggers the sharp surge in luteinising hormone that causes ovulation.

Once the corpus luteum's progesterone output is high enough, this progesterone temporarily suppresses further release of FSH and LH from the pituitary gland, preventing another follicle from maturing while the current cycle continues. When the corpus luteum breaks down and progesterone falls at the end of the cycle, this suppression is lifted, allowing FSH levels to rise again and start the next cycle.

How it is examined

A common question format shows a graph of hormone levels or uterus lining thickness over 28 days and asks you to identify the day of ovulation, name the phase at a given point, or explain what happens to hormone levels if fertilisation does or does not occur.

Worked exam-style question

Question. A graph shows the thickness of the uterus lining over a 28-day cycle. The lining is thin at day 1, breaks down over the first five days, then rises steadily from around day 5, reaches a peak thickness by around day 22, and then falls sharply back to the thin starting level by day 28.

(a) Name the phase occurring between day 1 and day 5. (b) Name the hormone mainly responsible for the rise in lining thickness between day 5 and day 14, and state where it is produced.

(c) Name the hormone mainly responsible for maintaining the thickened lining between day 15 and day 22, and state where it is produced. (d) Explain why the lining breaks down sharply just before day 28.

Model answer. (a) This is menstruation, when the previous cycle's thickened lining is shed. (b) The hormone is oestrogen, produced by the developing follicle in the ovary, which repairs and thickens the uterus lining as it prepares for possible implantation.

(c) The hormone is progesterone, produced by the corpus luteum, formed from the ruptured follicle after ovulation. (d) Because fertilisation did not occur, the corpus luteum breaks down near the end of the cycle, so progesterone levels fall sharply; without progesterone to maintain it, the thickened lining can no longer be sustained and is shed, starting menstruation again.

Practice question

Try this. A woman is trying to identify the most fertile days of her cycle. Explain, using the hormones FSH, LH, oestrogen and progesterone, which day of a typical 28-day cycle is most closely linked to the release of an egg, and why.

Exam tip

Key terms

Revise these linked terms to answer menstrual cycle questions precisely:

• Fertilisation, the event that determines whether the cycle continues to menstruation or is interrupted by pregnancy.
• Zygote, the cell formed if fertilisation occurs, which then signals the body to maintain the uterus lining.
• Placenta, the structure that later takes over hormone production if pregnancy is established.
• Hormone, the general term for the chemical messengers (FSH, LH, oestrogen, progesterone) that control the whole cycle.

Source:SRC-DSKP-EN

Frequently asked questions

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