Hormones in Human Reproduction - The Menstrual Cycle: Hormones (GCSE Biology)

The Menstrual Cycle: Hormones

The Menstrual Cycle

The menstrual cycle is the period of time in which an egg develops and is released, ready for fertilisation. The menstrual cycle takes around 28 days.

It has been described as the time scale from the beginning of a woman’s period, to the day before the next period. Many changes occur during the menstrual cycle, but its main role is to prepare an egg for release. It is controlled by hormones.

Steps of Menstrual Cycle

The key steps of the menstrual cycle is as follows. For the full names of the hormones see the section after:

1. Uterus lining breaks down. The uterus lining present is completely broken down. This takes around 3-4 days, and is known as the ‘period’. During this time bleeding occurs.
2. The egg starts to mature due to FSH. FSH is released by the pituitary gland into the bloodstream. It stimulates growth of the follicle in the ovary. The follicles then form the egg cells. These eggs can then mature.
3. The uterus lining builds up due to oestrogen. Oestrogen is released, which helps to develop the uterus lining again, in preparation for the new egg.
4. The egg is released due to high LH. Oestrogen increases the amount of LH, and decreases the amount of FSH. This causes the egg to be released from the follicle – a process called ovulation.
5. The uterus lining is maintained due to progesterone. After the egg is released from the follicle, the follicle produces progesterone. Progesterone helps to maintain the lining of the uterus, keeping it thick and intact for the egg. Progesterone at this stage reduces release of LH (LH has already done its job!) and FSH.
6. Fertilisation may occur. If sexual intercourse is had, a sperm can swim to the egg in the ovaries and fertilise it (fuse with it). This forms a fertilised egg, which can then be delivered to the uterus lining, where it will grow and develop into a zygote.
7. Fertilisation might not occur. If no sexual intercourse is had, then the level of progesterone falls around day 28, and the uterus lining breaks down. The cycle is complete, and starts again.

This table below corresponds with the 7 steps above. We have indicated where the hormone levels are high or low, but an empty box means that the levels are moderate (not zero).

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Summary of Hormones

You need to know about the four key hormones below.

Luteinising Hormone (LH)

• Released mainly during day 14
• Produced by pituitary gland
• Key in ovulation – oestrogen stimulates a surge in LH levels to cause release of the egg from the follicle

Follicle Stimulating Hormone (FSH)

• Released mainly during days 2-3
• Produced by pituitary gland
• Key in maturation of egg – it stimulates the egg to mature in the follicle

Oestrogen

• Released mainly during days 4-15
• Produced by ovaries
• Key in growing uterus lining – it helps the uterus lining grow and develop
• Key in ovulation – oestrogen stimulates a surge in LH (and fall in FSH) to cause release of the egg from the follicle

Progesterone

• Released mainly during days 15-27
• Produced by follicle in the ovaries
• Key in maintaining uterus lining – it prevents breakdown of the uterus lining from day 15 onwards
• Key in the end of the menstrual cycle – the fall in progesterone at day 28 leads to the breakdown of the uterus lining

The Hormones During Pregnancy

During pregnancy the hormones have different roles:

• FSH and LH are inactive
• Oestrogen and progesterone levels remain high – this is to maintain the uterus lining and to stimulate breast growth as well as the development of milk ducts.
• The placenta also produces oestrogen and progesterone