Edexcel IGCSE Biology · Spec 3.9
The Menstrual Cycle
The stages of the menstrual cycle and the changes in the ovaries and the lining of the uterus.
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The Menstrual Cycle
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Explained
The menstrual cycle, and the two halves either side of ovulation
The cycle lasts about 28 days, though it varies, and day 1 is counted from the first day of menstruation.
Two things are happening at once, and keeping them separate makes the whole cycle easier to follow. In the ovary an egg is maturing and then being released. In the uterus the lining is being rebuilt and then maintained, ready for an embryo that may or may not arrive.
The four events in order
Days 1 to 5, menstruation. The lining of the uterus breaks down and is lost through the vagina.
Days 5 to 14, the lining is repaired and thickens again, while an egg matures in the ovary.
Day 14, ovulation. The mature egg is released from the ovary into the oviduct. Ovulation happens in the ovary, not in the oviduct or the uterus, and that is a distinction questions test directly.
Days 14 to 28, the lining is maintained, thick and rich in blood vessels, in case a fertilised egg arrives.
If no fertilisation happens, the lining breaks down and day 1 comes round again. If it does, the lining stays and the cycle stops for the duration of the pregnancy.
Oestrogen
Oestrogen is produced by the ovary during the first half of the cycle, after menstruation ends.
It causes the lining of the uterus to repair and thicken, and its rising level is what triggers ovulation at around day 14. So oestrogen belongs to the first half, and its job is building.
Progesterone
Progesterone is produced after ovulation, during the second half of the cycle, by the structure the egg left behind in the ovary.
It maintains the lining that oestrogen built. When its level falls, near the end of the cycle, the lining can no longer be maintained and menstruation begins.
So the fall in progesterone is what causes a period, rather than anything actively breaking the lining down. During pregnancy progesterone stays high, the lining is kept, and no period happens. That single fact explains both the normal cycle and what changes when it stops.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks for the role of progesterone, for two marks, from three points: that it is released after ovulation or in the second half of the cycle, that it maintains the uterus lining or prevents menstruation, and that this allows an embryo to implant.
Its accept list is wide. It takes released from the corpus luteum, or during the luteal phase. It takes the idea that menstruation occurs when progesterone drops, or that the lining breaks down when it drops. It takes prepare for implantation, and it takes the point that progesterone reduces FSH and LH.
Then, in the middle of all that generosity, one word is refused: ignore thickens.
Thickening the lining is what oestrogen does in the first half of the cycle. Progesterone arrives after ovulation to a lining that is already thick, and its job is to keep it that way. Maintains and thickens sound like near synonyms in ordinary English, and here they belong to two different hormones acting at two different times.
Two further rulings are worth noting. It ignores egg if not fertilised, and it ignores baby. The structure that implants is an embryo, and the mark is for saying so.
The examiner report on that paper adds where else the marks went. Some candidates said progesterone repairs the lining, which is again the first half of the cycle. Some referred to the implantation of eggs rather than of an embryo. And some placed progesterone in the first half of the cycle altogether.
Every one of those errors is a matter of timing. Fix when each hormone acts and most of this question answers itself.
Answering a cycle question
Give the day or the phase, then the event, then the hormone responsible.
Where a graph of hormone levels is given, read across rather than guessing. The hormone that peaks just before day 14 triggers ovulation. The one that rises after day 14 and falls at the end is progesterone. The one that rises through the first half is oestrogen.
And take care with the words repair, thicken and maintain. They describe three stages of the same lining, and the mark schemes treat them as three different claims.
Spec 3.9
What you need to know
- Describe the main events of the menstrual cycle
- Explain the role of the hormone oestrogen
- Explain the role of the hormone progesterone
Active recall
Quick check
Answer each question before opening the answer.
What happens around day 14 of the menstrual cycle?
Ovulation — an egg is released from the ovary.
What happens to the uterus lining if the egg is not fertilised?
It breaks down and is lost through the vagina (menstruation).
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