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Unlock This CourseThe menstrual cycle is a monthly process that prepares a woman's body for pregnancy. Two key hormones - oestrogen and progesterone - work together like a perfectly timed orchestra to control this cycle. Understanding how these hormones work is crucial for IGCSE Biology and helps explain many aspects of human reproduction.
Key Definitions:
Oestrogen dominates the first half of the menstrual cycle (days 1-14). It's produced by developing follicles in the ovaries and causes the uterus lining to thicken in preparation for a potential pregnancy. Think of oestrogen as the 'builder' hormone - it builds up the endometrium and triggers ovulation when levels peak.
Progesterone takes over in the second half of the cycle (days 14-28). Produced by the corpus luteum, it maintains the thick uterus lining and prevents further ovulation. If pregnancy doesn't occur, progesterone levels drop dramatically, causing menstruation to begin.
Let's follow the incredible journey of hormones through a typical menstrual cycle, understanding exactly what happens when and why.
The cycle begins with menstruation - the shedding of the uterus lining. Both oestrogen and progesterone levels are at their lowest point. The pituitary gland responds by releasing FSH (Follicle Stimulating Hormone), which stimulates follicles in the ovaries to start developing.
Uterus lining sheds as menstrual flow. Low hormone levels signal the brain to restart the cycle.
Oestrogen: Very low. Progesterone: Very low. FSH: Rising to stimulate new follicles.
This phase resets the cycle and prepares for the next potential pregnancy.
As follicles develop, they start producing increasing amounts of oestrogen. This hormone causes the endometrium (uterus lining) to thicken and become rich with blood vessels. Oestrogen levels rise steadily, preparing the body for ovulation.
During this phase, oestrogen also changes cervical mucus to become thinner and more alkaline, creating a better environment for sperm survival. This shows how hormones coordinate multiple body systems!
When oestrogen reaches its peak level, it triggers a surge of LH (Luteinising Hormone) from the pituitary gland. This LH surge causes the dominant follicle to release its mature egg - this is ovulation. The egg travels into the fallopian tube where it could potentially be fertilised.
Oestrogen reaches its highest level just before ovulation. This peak is crucial - it switches from inhibiting LH to stimulating a massive LH surge that triggers egg release.
The LH surge lasts about 24-36 hours and is so dramatic that ovulation predictor kits can detect it in urine. This surge transforms the follicle into the corpus luteum.
After ovulation, the empty follicle becomes the corpus luteum, which produces high levels of progesterone and some oestrogen. Progesterone maintains the thick endometrium and prevents further ovulation. If the egg isn't fertilised, the corpus luteum breaks down around day 26, causing hormone levels to plummet and triggering menstruation.
The menstrual cycle demonstrates beautiful examples of negative feedback - a key concept in biology where the body maintains balance by responding to changes.
When oestrogen levels are low, the pituitary releases FSH to stimulate follicle development. As follicles grow and produce more oestrogen, this high oestrogen eventually inhibits FSH production - preventing too many eggs from developing at once.
Interestingly, very high oestrogen levels switch from negative to positive feedback, triggering the LH surge. This is one of the few examples of positive feedback in human biology!
Sarah, 16, has irregular periods ranging from 21-35 days. Her doctor explains this is normal for teenagers as their hormone systems are still maturing. The hypothalamus-pituitary-ovary axis takes time to establish regular patterns. Factors like stress, weight changes and exercise can affect hormone production, leading to cycle variations. Understanding normal hormone fluctuations helps distinguish between typical variations and conditions requiring medical attention.
Understanding oestrogen and progesterone has led to important medical advances that affect millions of women worldwide.
The combined contraceptive pill contains synthetic versions of oestrogen and progesterone. These hormones prevent ovulation by maintaining consistently high hormone levels, tricking the body into thinking it's already pregnant. The pill also thickens cervical mucus and thins the endometrium as backup contraceptive mechanisms.
Synthetic hormones prevent the natural hormone fluctuations needed for ovulation to occur.
When used correctly, the combined pill is over 99% effective at preventing pregnancy.
The pill doesn't protect against sexually transmitted infections - only barrier methods like condoms do.
Doctors use hormone knowledge to help couples struggling with infertility. Treatments might involve giving FSH injections to stimulate multiple follicles, or using progesterone supplements to maintain the uterus lining after embryo transfer in IVF procedures.
During menopause, ovaries stop producing oestrogen and progesterone, causing symptoms like hot flushes and bone loss. HRT replaces these hormones to manage symptoms and protect against osteoporosis.
The discovery of how oestrogen and progesterone work led to the development of the contraceptive pill in the 1960s. This gave women unprecedented control over their reproductive choices and is considered one of the most significant medical advances of the 20th century. The scientists who developed it were inspired by observing that pregnant women don't ovulate - leading them to use pregnancy hormones to prevent pregnancy!
Let's clear up some common misunderstandings about menstrual cycle hormones:
Reality: Normal cycles range from 21-35 days. The 28-day cycle is just an average - many healthy women have shorter or longer cycles.
Reality: Ovulation timing varies with cycle length. It typically occurs 14 days before the next period, not 14 days after the previous one.
Understanding oestrogen and progesterone helps explain not just reproduction, but also why women might experience mood changes, skin changes, or appetite variations throughout their cycle. These hormones affect many body systems beyond just the reproductive organs, showing how interconnected our biology really is.