The Uterus Rebuilds Its Lining Every Month in Case a Baby Needs It
Follow the menstrual cycle phase by phase and find when ovulation happens, learn what oestrogen and progesterone do in the cycle and in pregnancy, see the roles of testosterone and prolactin, and understand menarche and menopause.
Why does the female body go through a monthly cycle?
Roughly once a month, an ovary releases an ovum and the uterus prepares a thick, blood-rich lining ready to receive an embryo. If no pregnancy begins, the lining is shed and the whole process starts again. This repeating sequence is the menstrual cycle.
This part covers the phases of the cycle, the hormones that drive it, the male hormone testosterone and the milk hormone prolactin, and the start and end of the reproductive years.
This part covers the phases of the cycle, the hormones that drive it, the male hormone testosterone and the milk hormone prolactin, and the start and end of the reproductive years.
What happens in each phase of the menstrual cycle, and when does ovulation occur?
The menstrual cycle lasts about 28 days: the uterine lining is shed during menstruation, rebuilt as a follicle ripens, an ovum is released at ovulation around day 14, and the lining is then maintained ready for pregnancy.
Phases of a 28-day cycle — day 1 is the first day of bleeding:
- Menstrual phase (days 1–5) — the thick lining of the uterus breaks down and leaves the body with some blood
- Follicular phase (days 6–13) — a follicle ripens in the ovary, and the uterine lining is repaired and thickened
- Ovulation (around day 14) — the mature follicle bursts and releases an ovum into the fallopian tube
- Luteal phase (days 15–28) — the empty follicle becomes the corpus luteum, and the lining becomes thick, soft and rich in blood vessels
If the ovum is not fertilised, the corpus luteum breaks down, hormone levels fall and the lining is shed — menstruation begins again.
Worked example. Ovulation usually happens about days before the next period. For a cycle of days:
An everyday example. Many schools now keep sanitary pads available for students, because understanding and managing periods is a normal part of health.
The substance. Cycle length varies between healthy women, so ovulation is not fixed on day 14 — it is counted back from the next period.
Phases of a 28-day cycle — day 1 is the first day of bleeding:
- Menstrual phase (days 1–5) — the thick lining of the uterus breaks down and leaves the body with some blood
- Follicular phase (days 6–13) — a follicle ripens in the ovary, and the uterine lining is repaired and thickened
- Ovulation (around day 14) — the mature follicle bursts and releases an ovum into the fallopian tube
- Luteal phase (days 15–28) — the empty follicle becomes the corpus luteum, and the lining becomes thick, soft and rich in blood vessels
If the ovum is not fertilised, the corpus luteum breaks down, hormone levels fall and the lining is shed — menstruation begins again.
Worked example. Ovulation usually happens about days before the next period. For a cycle of days:
An everyday example. Many schools now keep sanitary pads available for students, because understanding and managing periods is a normal part of health.
The substance. Cycle length varies between healthy women, so ovulation is not fixed on day 14 — it is counted back from the next period.
What roles do oestrogen and progesterone play in the menstrual cycle and in pregnancy?
Oestrogen rebuilds the uterine lining after menstruation and brings about female secondary sexual characters, while progesterone prepares and maintains the lining for an embryo and keeps a pregnancy going.
Oestrogen — secreted by the growing follicle in the ovary
- Repairs and thickens the uterine lining after menstruation
- Develops and maintains female secondary sexual characters
- In pregnancy, supports the growth of the uterus and breasts
Progesterone — secreted by the corpus luteum
- Makes the lining thicker and richer in glands and blood vessels, ready for implantation
- Maintains the lining; when progesterone falls, menstruation starts
- In pregnancy, keeps the uterus lining in place, stops further ovulation and keeps uterine muscles calm
During pregnancy, the corpus luteum keeps working at first; later the placenta takes over making both hormones.
An everyday example. A home pregnancy test detects a hormone made by the developing embryo's tissues — the signal that keeps the corpus luteum producing progesterone.
The substance. Progesterone is often called the pregnancy hormone, because without it the lining would be shed and the embryo lost.
Oestrogen — secreted by the growing follicle in the ovary
- Repairs and thickens the uterine lining after menstruation
- Develops and maintains female secondary sexual characters
- In pregnancy, supports the growth of the uterus and breasts
Progesterone — secreted by the corpus luteum
- Makes the lining thicker and richer in glands and blood vessels, ready for implantation
- Maintains the lining; when progesterone falls, menstruation starts
- In pregnancy, keeps the uterus lining in place, stops further ovulation and keeps uterine muscles calm
During pregnancy, the corpus luteum keeps working at first; later the placenta takes over making both hormones.
An everyday example. A home pregnancy test detects a hormone made by the developing embryo's tissues — the signal that keeps the corpus luteum producing progesterone.
The substance. Progesterone is often called the pregnancy hormone, because without it the lining would be shed and the embryo lost.
What roles do testosterone and prolactin play in reproduction?
Testosterone from the testes drives sperm production and male secondary sexual characters, while prolactin from the pituitary makes the breasts produce milk after childbirth.
Testosterone — secreted by cells between the sperm-producing tubules of the testes
- Stimulates sperm production
- Develops male secondary sexual characters — deeper voice, facial hair, muscular build
- Maintains the male accessory glands
Prolactin — secreted by the anterior pituitary
- Stimulates the mammary glands to produce milk after childbirth
- Its level rises while a baby is breastfed, keeping milk production going
An everyday example. The first thick, yellowish milk a mother produces, colostrum, is rich in antibodies, which is why doctors encourage feeding it to newborns.
The link. Prolactin makes milk, but oxytocin, from the posterior pituitary, makes it flow out when the baby suckles — two pituitary hormones sharing one job.
Testosterone — secreted by cells between the sperm-producing tubules of the testes
- Stimulates sperm production
- Develops male secondary sexual characters — deeper voice, facial hair, muscular build
- Maintains the male accessory glands
Prolactin — secreted by the anterior pituitary
- Stimulates the mammary glands to produce milk after childbirth
- Its level rises while a baby is breastfed, keeping milk production going
An everyday example. The first thick, yellowish milk a mother produces, colostrum, is rich in antibodies, which is why doctors encourage feeding it to newborns.
The link. Prolactin makes milk, but oxytocin, from the posterior pituitary, makes it flow out when the baby suckles — two pituitary hormones sharing one job.
What are menarche and menopause, and at what age does each usually occur?
Menarche is the first menstruation, usually between about 11 and 14 years of age, and menopause is the permanent end of menstruation, usually between about 45 and 50 years of age.
Menarche
- The first menstrual period, marking the start of the reproductive years
- Early cycles are often irregular while hormone levels settle
Menopause
- The ovaries stop releasing ova, and oestrogen and progesterone fall
- Menstruation stops permanently; some women feel sudden warmth, called hot flushes, or mood changes
Worked example. Suppose menarche is at and menopause at , with a cycle of days:
**So at most about ova are released in this example, and fewer if there are pregnancies.
An everyday example. An older aunt who mentions that her periods have stopped is describing menopause, a natural stage of life, not an illness.
The substance. Menarche and menopause both vary between healthy women**, and a few years earlier or later than the typical range is usually normal.
Menarche
- The first menstrual period, marking the start of the reproductive years
- Early cycles are often irregular while hormone levels settle
Menopause
- The ovaries stop releasing ova, and oestrogen and progesterone fall
- Menstruation stops permanently; some women feel sudden warmth, called hot flushes, or mood changes
Worked example. Suppose menarche is at and menopause at , with a cycle of days:
**So at most about ova are released in this example, and fewer if there are pregnancies.
An everyday example. An older aunt who mentions that her periods have stopped is describing menopause, a natural stage of life, not an illness.
The substance. Menarche and menopause both vary between healthy women**, and a few years earlier or later than the typical range is usually normal.
Exam tip
What earns full marks on the menstrual cycle and reproductive hormones?
Name each phase with its days and events, match each hormone to its source and role, and state the ages of menarche and menopause as approximate ranges.
- Phases: menstrual, follicular, ovulation around day 14, luteal
- Corpus luteum — forms after ovulation and secretes progesterone
- Oestrogen from follicle; progesterone from corpus luteum and later placenta
- Testosterone from testes; prolactin from anterior pituitary
- Menarche about 11 to 14; menopause about 45 to 50
The trap. Writing that menstruation happens because oestrogen falls. It is the fall in progesterone after the corpus luteum breaks down that makes the lining shed.
- Phases: menstrual, follicular, ovulation around day 14, luteal
- Corpus luteum — forms after ovulation and secretes progesterone
- Oestrogen from follicle; progesterone from corpus luteum and later placenta
- Testosterone from testes; prolactin from anterior pituitary
- Menarche about 11 to 14; menopause about 45 to 50
The trap. Writing that menstruation happens because oestrogen falls. It is the fall in progesterone after the corpus luteum breaks down that makes the lining shed.
Did you know
Why is a girl born with every egg cell she will ever have?
Unlike sperm, which are made continuously from puberty onwards, egg cells are not made afresh through life.
Before a girl is born, her ovaries already hold a very large store of immature egg cells. Most of them gradually break down over the years without ever being released.
Only a few hundred ever mature and are released — which matches the lifetime count worked out above. Menopause comes when this store is almost used up.
Before a girl is born, her ovaries already hold a very large store of immature egg cells. Most of them gradually break down over the years without ever being released.
Only a few hundred ever mature and are released — which matches the lifetime count worked out above. Menopause comes when this store is almost used up.
Exam relevance
How does the menstrual cycle lead into NEET Biology?
This is foundation work for Class 12 Human Reproduction and Reproductive Health in NEET Biology.
What gets built on. Human Reproduction adds the pituitary hormones FSH and LH to the cycle, explains that a surge of LH triggers ovulation, and presents the changing levels of all four hormones on a graph across the cycle. It also covers the placenta as a hormone source in pregnancy. Reproductive Health links these hormones to contraceptive methods.
Question types. Graph-based questions reading hormone levels against the day of the cycle, and statement questions on the phases.
The trap that costs marks. Saying oestrogen directly causes ovulation. The LH surge triggers ovulation, and the corpus luteum then secretes progesterone.
What gets built on. Human Reproduction adds the pituitary hormones FSH and LH to the cycle, explains that a surge of LH triggers ovulation, and presents the changing levels of all four hormones on a graph across the cycle. It also covers the placenta as a hormone source in pregnancy. Reproductive Health links these hormones to contraceptive methods.
Question types. Graph-based questions reading hormone levels against the day of the cycle, and statement questions on the phases.
The trap that costs marks. Saying oestrogen directly causes ovulation. The LH surge triggers ovulation, and the corpus luteum then secretes progesterone.
Key takeaways
What must you be able to do from this part?
- Cycle of about 28 days: menstrual phase, follicular phase, ovulation, luteal phase
- Ovulation about 14 days before the next period — around day 14, or day 16 in a 30-day cycle
- Corpus luteum forms after ovulation; breaks down if no fertilisation
- Oestrogen: rebuilds lining, female secondary characters
- Progesterone: prepares and maintains lining, keeps pregnancy going
- Testosterone: sperm production, male secondary characters
- Prolactin: milk production; oxytocin makes milk flow
- Menarche about 11 to 14 years; menopause about 45 to 50 years
Draw a single 28-day strip and see if you can mark every phase and the hormone in charge without looking back.
- Ovulation about 14 days before the next period — around day 14, or day 16 in a 30-day cycle
- Corpus luteum forms after ovulation; breaks down if no fertilisation
- Oestrogen: rebuilds lining, female secondary characters
- Progesterone: prepares and maintains lining, keeps pregnancy going
- Testosterone: sperm production, male secondary characters
- Prolactin: milk production; oxytocin makes milk flow
- Menarche about 11 to 14 years; menopause about 45 to 50 years
Draw a single 28-day strip and see if you can mark every phase and the hormone in charge without looking back.