Hormones and the Menstrual Cycle

During puberty, reproductive hormones cause secondary sex characteristics to develop
The main male reproductive hormone is testosterone which is produced by the testes; testosterone stimulates sperm production
The main female reproductive hormone is oestrogen which is produced by the ovaries
Oestrogen plays an important role in the menstrual cycle which begins at puberty, as well as causing physical changes to occur in the body (such as breast development)
During the menstrual cycle, eggs in the ovaries begin to mature and one is released approximately every 28 days in a process called ovulation

During the menstrual cycle, the lining of the uterus builds up and ovulation occurs
The average menstrual cycle is 28 days long and there are four overall stages:
1. Menstruation – loss of lining from the uterus, occurs at the start of the cycle if no fertilisation has occurred
2. The lining starts to thicken
3. Ovulation occurs around the middle of the cycle (about day 14), the egg travels down the oviduct towards the uterus
4. The lining is maintained ready to accept a fertilized egg

Menstruation occurs at the start of the menstrual cycle, with an egg released around the middle

Four hormones control the events that occur during the menstrual cycle
Two of these hormones are produced by the pituitary gland in the brain:
- Follicle-stimulating hormone (FSH) causes maturation of an egg in the ovary
- Luteinising hormone (LH) stimulates the release of the egg
The other two hormones, oestrogen and progesterone are involved in maintaining the uterus lining with oestrogen being made by the ovaries and progesterone specifically by an empty egg follicle called the corpus luteum

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The pituitary gland produces FSH which stimulates the development of a follicle in the ovary
An egg matures inside the follicle and the follicle produces the hormone oestrogen – so FSH stimulates the production of oestrogen
Oestrogen causes growth and repair of the lining of the uterus wall and inhibits the production of FSH
When oestrogen rises to a high enough level it stimulates the release of LH from the pituitary gland which causes ovulation (usually around day 14 of the cycle)
The follicle becomes a corpus luteum and starts producing progesterone
Progesterone maintains the uterus lining (the thickness of the uterus wall)
If the egg is not fertilised, the corpus luteum breaks down and progesterone levels drop
This causes menstruation – commonly known as having a period
If fertilisation does occur the corpus luteum continues to produce progesterone, preventing the uterus lining from breaking down (breakdown of the lining would prevent a pregnancy)
Once the placenta has developed, it starts secreting progesterone and continues to do so throughout the pregnancy to maintain the lining

Remember that hormones travel around the body in the bloodstream but only have an effect on a target organ

You need to be able to extract and interpret data from graphs showing hormone levels during the menstrual cycle:

Changes in the levels of the pituitary hormones FSH and LH in the blood during the menstrual cycle

FSH (follicle-stimulating hormone) is released by the pituitary gland and causes an egg to start maturing in the ovary
- It also stimulates the ovaries to start releasing oestrogen
LH (luteinising hormone) is released from the pituitary gland
- This is stimulated when oestrogen levels have reached their peak
- LH causes ovulation to occur and also stimulates the ovary to produce progesterone

Changes in the levels of oestrogen and progesterone in the blood during the menstrual cycle

Oestrogen levels rise from day 1 to peak just before day 14
- This causes the uterine wall to start thickening and the egg to mature
- The peak in oestrogen occurs just before the egg is released
Progesterone stays low from day 1 – 14 and starts to rise after ovulation
- The increasing levels cause the uterine lining to thicken further; a fall in progesterone levels causes the uterine lining to break down (menstruation)