IGCSE Biology / Reproduction in humans
Hormones and the menstrual cycle revision notes
Edexcel International GCSE Biology (4BI1), chapter 9: Reproduction in humans, specification point 3.9, 3.10B. Free lesson.
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The menstrual cycle is controlled by four hormones that change in level over about 28 days. Knowing where each is made and what it does helps you explain ovulation, menstruation and what happens after fertilisation.
What the hormone graph shows
The graph tracks hormone levels across a 28-day cycle. Oestrogen rises during the first half and peaks just before day 14. This peak triggers a surge of LH, which causes ovulation at about day 14. Progesterone rises after day 14 and peaks around day 21. If there is no pregnancy, it then falls sharply. The fall means the uterus lining is no longer maintained, so menstruation begins. Ovulation happens at about day 14, when the mature follicle releases its egg. Menstruation follows the fall in progesterone at the end of the cycle.
What FSH and oestrogen do
Two of the hormones are made by the pituitary gland, and two are made by the ovary. FSH starts the process by helping a follicle mature. The follicle is a structure in the ovary that contains an egg. FSH is made by the pituitary. It matures a follicle in the ovary and stimulates the ovary to produce oestrogen. Oestrogen is made by the ovary. It rebuilds and thickens the uterus lining. Oestrogen also inhibits FSH, which helps ensure that only one follicle matures.
What LH and progesterone do
The other two hormones act at different points in the cycle. A rise in LH releases the egg, while progesterone keeps the uterus lining ready after ovulation. LH is made by the pituitary. A surge in LH causes ovulation. Progesterone is made by the remains of the follicle in the ovary. It maintains the thickened uterus lining. Progesterone is not made by the pituitary or by the uterus.
What happens with or without fertilisation
After ovulation, the level of progesterone depends on whether fertilisation occurs. This determines whether the uterus lining is kept or breaks down. If there is no fertilisation, progesterone falls. The lining breaks down and menstruation begins. If fertilisation occurs, progesterone stays high and the lining is maintained, so a period does not come.
Keep each hormone's job separate
It is easy to mix up which hormone affects the uterus lining. Remember that each has a separate job, so naming the wrong hormone can lose marks. FSH matures a follicle and stimulates oestrogen production. Oestrogen rebuilds and thickens the uterus lining. LH triggers ovulation. Progesterone maintains the lining, and its fall causes menstruation.