Biology · Hormones and Reproduction
The Menstrual Cycle
About once a month, from puberty until the menopause, the body of a woman gets ready for a possible pregnancy. This lesson covers the stages of the menstrual cycle, and the roles of the hormones oestrogen and progesterone in controlling it.
Learning Objectives
- 1Describe the stages of the menstrual cycle, including menstruation, the repair of the uterus lining, ovulation and the maintenance of the lining.
- 2Describe the roles of the hormones oestrogen and progesterone in the control of the menstrual cycle.
- 3State the days of the cycle on which each stage takes place.
- 4Interpret information about the menstrual cycle in a diagram or graph.
Retrieval practice
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1
What is an ovary?
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An organ that makes eggs, and the hormones oestrogen and progesterone.
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2
What is fertilisation?
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The fusion of a sperm and an egg cell.
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3
What is a hormone?
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A chemical messenger that is made in a gland and carried in the blood.
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4
What is the uterus?
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The organ in which an embryo develops.
A monthly cycle
From puberty, which starts at about the age of 10 to 14, a woman's reproductive system goes through a regular cycle, which is called the menstrual cycle. It lasts about 28 days, though it can be shorter or longer. In each cycle, one egg matures and is released from an ovary, and the lining of the uterus gets ready to receive an embryo. If there is no pregnancy, the lining is lost, and the cycle starts again. The stages are controlled by hormones, and oestrogen and progesterone, made by the ovaries, have key roles.
The stages of the cycle
The cycle is counted from the first day of bleeding, which is day 1.
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Days 1 to 5: menstruation
The lining of the uterus breaks down, and leaves the body through the vagina, with a small amount of blood. This is a period.
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Days 6 to 13: repair
The lining of the uterus is repaired and gets thicker, with a good supply of blood vessels. An egg matures in a follicle in an ovary.
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Day 14: ovulation
A mature egg is released from the ovary, and passes into the oviduct. This is ovulation.
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Days 15 to 28: maintenance
The lining of the uterus is kept thick and full of blood vessels, in case a fertilised egg arrives. If fertilisation does not happen, the lining breaks down on about day 28, and a new cycle begins.
The stages of the menstrual cycle
The menstrual cycle has four stages, which are controlled by hormones.
The roles of oestrogen and progesterone
These two hormones are made by the ovaries.
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Oestrogen
It is made by the ovary, by the follicle that contains the maturing egg. It causes the lining of the uterus to be repaired, and to thicken, after a period. It also causes the development of female features at puberty.
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Progesterone
It is made by the ovary, in the empty follicle after ovulation. It maintains the thick lining of the uterus, which is ready for an embryo.
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Falling levels
If there is no pregnancy, the level of progesterone falls. The lining can no longer be maintained, and breaks down, which is menstruation.
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Pregnancy
If an egg is fertilised, it implants in the lining. The progesterone level stays high, so the lining is maintained, and there is no period.
Oestrogen and progesterone
Oestrogen
- Made by the ovary
- Rises in the first half of the cycle
- Repairs and thickens the lining of the uterus
- Causes female features at puberty
Progesterone
- Made by the ovary after ovulation
- Rises in the second half of the cycle
- Maintains the lining of the uterus
- If it falls, the lining breaks down
Working out the dates of a cycle
A woman has a regular 28-day menstrual cycle, and her last period began on 1 March. State the date on which she is likely to ovulate, and the date on which her next period is likely to start.
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- 1 Day 1 1 March is day 1 of the cycle.
- 2 Ovulation It occurs on day 14, which is 14 March.
- 3 The next period The cycle lasts 28 days, so day 29 is day 1 of the next cycle, which is 29 March.
- 4 Check March has 31 days, so day 29 of the cycle is 29 March.
AnswerShe is likely to ovulate on 14 March, and her next period is likely to start on 29 March.
Reading a diagram of the cycle
Questions often show the thickness of the lining, or the levels of hormones, against the days of the cycle.
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Thickness of the lining
It is thin at days 1 to 5, grows from day 6, and is thickest on days 15 to 28.
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Ovulation
Look for day 14, near the middle of the cycle.
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Oestrogen
It rises through days 6 to 13, then falls just after ovulation.
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Progesterone
It is low until ovulation, and then high, until about day 25.
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Matching
Match each change in the lining to the hormone that is rising at that time.
Case study
Tracking the cycle
Many women use a calendar or a phone app to record the days of their cycle. Because ovulation happens about 14 days before the next period, and a sperm can live for several days in the body, a woman is most likely to become pregnant if she has sex in the few days before, and just after, ovulation. Some people use this to plan a pregnancy, or to avoid one, but a cycle that is longer or shorter than 28 days, or that varies from month to month, makes it hard to predict, so the method is not reliable for contraception.
Summary and exam focus
- The menstrual cycle lasts about 28 days, starting with menstruation.
- The lining of the uterus is repaired and thickened after the period.
- Ovulation is the release of an egg, on about day 14.
- Progesterone maintains the lining of the uterus.
- Oestrogen and progesterone are made by the ovaries.
Exam focus
Describe the stages of the menstrual cycle, and the roles of oestrogen and progesterone in controlling it. (6 marks) (6 marks)
Go through the cycle in order, giving the days: menstruation on days 1 to 5, repair of the lining, ovulation on day 14, and maintenance of the lining. Then say that oestrogen causes the lining to repair and thicken, and that progesterone maintains the lining.
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Menstrual cycle
- The monthly cycle in the female reproductive system.
- Menstruation
- The loss of the lining of the uterus, from the vagina, at the start of each cycle.
- Ovulation
- The release of a mature egg from an ovary.
- Oestrogen
- A hormone from the ovaries that repairs and thickens the lining of the uterus.
- Progesterone
- A hormone from the ovaries that maintains the lining of the uterus.
- Uterus
- The organ in which an embryo grows.
- Follicle
- A structure in the ovary in which an egg matures.
- Puberty
- The stage when the sex organs become mature.
- Oviduct
- The tube that carries an egg from the ovary to the uterus.
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