What you'll learn
Hormonal control and the menstrual cycle explains how four hormones interact to produce a repeating monthly cycle, and how that interaction is regulated by feedback. It is one of the best examples in the syllabus of a system controlled by both negative and positive feedback, and questions frequently turn on identifying which is operating at a given point. At CAPE level you must be able to follow the concentration of each hormone through the cycle, explain what causes each rise and fall, relate hormone concentrations to events in the ovary and uterus, and apply the same understanding to contraception and fertility treatment. By the end of this topic you should be able to name the source and target of each hormone, describe the events of the cycle in sequence, distinguish the feedback mechanisms, interpret hormone concentration graphs, and explain the hormonal basis of contraception and infertility treatment.
Key terms and definitions
Menstrual cycle — the approximately 28-day cycle of changes in the ovary and uterus
Follicle stimulating hormone — the pituitary hormone stimulating follicle development, abbreviated FSH
Luteinising hormone — the pituitary hormone triggering ovulation, abbreviated LH
Oestrogen — the hormone secreted by the developing follicle, which repairs and thickens the endometrium
Progesterone — the hormone secreted by the corpus luteum, which maintains the endometrium
Follicular phase — the first half of the cycle, during which a follicle develops
Ovulation — the release of a secondary oocyte from the Graafian follicle
Luteal phase — the second half of the cycle, dominated by the corpus luteum
Corpus luteum — the structure formed from the ruptured follicle, secreting progesterone
Endometrium — the lining of the uterus, which thickens and is shed each cycle
Negative feedback — a response that reverses the change that produced it
Positive feedback — a response that amplifies the change that produced it
Human chorionic gonadotrophin — the hormone from an implanted embryo that maintains the corpus luteum, abbreviated hCG
Core concepts
The four hormones and their sources
Two hormones come from the anterior pituitary gland and two from the ovary, and keeping the sources straight is the foundation of the whole topic.
Follicle stimulating hormone is secreted by the anterior pituitary. It stimulates the development of follicles in the ovary and stimulates those follicles to secrete oestrogen.
Luteinising hormone is also secreted by the anterior pituitary. A sharp surge in its concentration triggers ovulation, and afterwards it stimulates the remains of the follicle to develop into the corpus luteum.
Oestrogen is secreted by the developing follicle. It causes the endometrium to repair and thicken after menstruation, and it acts on the pituitary — inhibiting FSH at low concentrations but stimulating a surge of LH at high concentrations.
Progesterone is secreted by the corpus luteum. It maintains the thickened endometrium and inhibits both FSH and LH.
The pituitary itself is controlled by gonadotrophin releasing hormone from the hypothalamus, which links the system to the brain.
The cycle in sequence
Taking day one as the first day of menstruation, the cycle proceeds as follows.
During menstruation, days one to about five, the concentrations of oestrogen and progesterone are low. Without progesterone to maintain it, the endometrium breaks down and is shed. The low concentrations also remove the inhibition of the pituitary, so FSH secretion rises.
In the follicular phase, days about six to thirteen, rising FSH stimulates several follicles to develop, of which usually one becomes dominant. The developing follicle secretes increasing quantities of oestrogen. Oestrogen causes the endometrium to repair and thicken, becoming rich in blood vessels and glands in preparation for a possible implantation. At the same time, moderate oestrogen concentrations inhibit FSH secretion by negative feedback, which prevents further follicles from developing and is why usually only one egg is released.
Around day twelve to thirteen, oestrogen from the now mature follicle reaches a high concentration. Above a threshold, its effect on the pituitary reverses: instead of inhibiting, it now stimulates a sharp surge in LH secretion, with a smaller rise in FSH. This switch from negative to positive feedback is the pivotal event of the cycle and the point examiners most often test.
At about day fourteen, the LH surge causes the mature Graafian follicle to rupture and release the secondary oocyte. This is ovulation.
In the luteal phase, days about fifteen to twenty-eight, LH stimulates the remains of the follicle to develop into the corpus luteum, which secretes progesterone together with some oestrogen. Progesterone maintains the thickened endometrium and inhibits both FSH and LH by negative feedback, preventing the development of further follicles while a possible pregnancy is being established.
If fertilisation does not occur, the corpus luteum degenerates after about ten days. Progesterone and oestrogen concentrations therefore fall. The endometrium can no longer be maintained and is shed as menstruation, and the removal of inhibition allows FSH to rise once more, beginning the next cycle.
If fertilisation and implantation do occur, the developing embryo secretes human chorionic gonadotrophin. This maintains the corpus luteum, so progesterone secretion continues, the endometrium is retained and menstruation does not occur. After about twelve weeks the placenta takes over the secretion of progesterone. It is hCG in the urine that pregnancy tests detect.
Negative and positive feedback in the cycle
Both mechanisms operate, and distinguishing them is a standard question.
Negative feedback operates for most of the cycle. Moderate oestrogen concentrations in the follicular phase inhibit FSH, which prevents additional follicles developing. Progesterone in the luteal phase inhibits both FSH and LH, which prevents a new cycle beginning while the endometrium is being maintained. In each case the hormone acts to reduce the stimulus that produced it.
Positive feedback operates briefly at mid-cycle. When oestrogen exceeds a threshold concentration, it stimulates rather than inhibits LH secretion. The resulting LH surge is rapid and self-reinforcing, and it triggers ovulation. This is amplification rather than reversal, which is what makes it positive feedback.
The functional logic is worth stating: negative feedback keeps the system stable and ensures a single follicle matures, while positive feedback produces the rapid, decisive surge needed to trigger a discrete event at a precise time.
Interpreting hormone graphs
Graphs of the four hormone concentrations against day of cycle are examined regularly, and the interpretation follows from a few landmarks.
FSH shows a modest early rise and a smaller mid-cycle peak alongside the LH surge.
LH shows a single dramatic spike at about day fourteen, which identifies ovulation and therefore the position of every other event.
Oestrogen rises through the follicular phase to a peak just before the LH surge, falls briefly, then shows a smaller second rise during the luteal phase from the corpus luteum.
Progesterone remains low through the follicular phase, rises after ovulation to a peak at about day twenty-one, then falls before menstruation.
The practical method for such questions is to locate the LH spike first, mark it as day fourteen and as ovulation, then identify everything before it as follicular and everything after as luteal.
Body temperature also rises slightly, by around 0.5 degrees Celsius, just after ovulation because of progesterone, which is the basis of one natural method of fertility awareness.
Hormonal contraception
Combined oral contraceptives contain synthetic oestrogen and progesterone. Their sustained presence inhibits FSH and LH secretion by negative feedback, so no follicle matures and no LH surge occurs, and ovulation is therefore prevented. The progesterone component also thickens cervical mucus, hindering sperm passage, and makes the endometrium less receptive to implantation.
Progesterone-only methods, including the mini-pill, injections and implants, work mainly through the cervical mucus and endometrial effects, and may also inhibit ovulation in some users.
Emergency contraception delivers a high dose of progesterone that delays or prevents ovulation.
Evaluation questions expect both sides. Hormonal methods are highly effective when used correctly, are reversible, and may reduce menstrual pain and the risk of certain cancers. Against this, they require consistent use, may cause side effects including nausea, mood changes and a small increase in the risk of thrombosis, and provide no protection against sexually transmitted infections, which barrier methods do.
Fertility treatment
Understanding the cycle explains how infertility is treated hormonally.
Where a woman does not ovulate because of insufficient FSH, drugs that block oestrogen receptors in the pituitary can be given. Since the pituitary no longer detects the oestrogen that would normally inhibit it, FSH secretion rises and follicle development is stimulated.
FSH itself may be administered directly to stimulate follicle development, which is also the basis of the superovulation used in in vitro fertilisation, where multiple follicles are deliberately stimulated so that several oocytes can be collected.
In vitro fertilisation then involves collecting the oocytes, fertilising them with sperm in the laboratory, culturing the resulting embryos for a few days, and transferring one or more to the uterus, with progesterone given to support the endometrium.
The risks include multiple pregnancy where several embryos are transferred, and ovarian hyperstimulation syndrome from the drugs used. Ethical considerations include the fate of surplus embryos and the cost and accessibility of treatment.
Worked examples
Example 1: Explaining the LH surge (5 marks)
Explain why luteinising hormone concentration rises sharply at about day fourteen of the cycle.
During the follicular phase the developing follicle secretes increasing quantities of oestrogen as it matures.
At moderate concentrations oestrogen inhibits the anterior pituitary by negative feedback, reducing FSH secretion and so preventing further follicles from developing.
By about day twelve the mature follicle is secreting oestrogen at a high concentration. Above a threshold value, the effect of oestrogen on the pituitary reverses: it now stimulates rather than inhibits the secretion of luteinising hormone.
This is positive feedback, because the response amplifies the original change rather than reversing it, producing a rapid and pronounced surge in LH concentration.
The LH surge causes the mature Graafian follicle to rupture and release the secondary oocyte, so ovulation occurs at about day fourteen.
Example 2: Explaining the effect of the contraceptive pill (4 marks)
Explain how a combined oral contraceptive prevents pregnancy.
The pill contains synthetic oestrogen and progesterone, which maintain a sustained high concentration of these hormones in the blood.
These hormones inhibit the secretion of FSH and LH by the anterior pituitary through negative feedback.
Without sufficient FSH, no follicle develops and matures in the ovary. Without the surge of LH, ovulation is not triggered, so no secondary oocyte is released and fertilisation cannot occur.
The progesterone component also thickens the mucus at the cervix, which impedes the passage of sperm, and makes the endometrium thinner and less receptive, so that implantation would be unlikely even if an oocyte were released and fertilised.
Example 3: Explaining what happens after implantation (4 marks)
Explain why menstruation does not occur if an embryo implants.
Normally the corpus luteum degenerates about ten days after ovulation. Progesterone concentration then falls, the endometrium can no longer be maintained, and it breaks down and is shed as menstruation.
If an embryo implants, it secretes human chorionic gonadotrophin. This hormone maintains the corpus luteum rather than allowing it to degenerate.
The corpus luteum therefore continues to secrete progesterone, which maintains the thickened, vascular endometrium so that it is not shed and the developing embryo remains implanted. The continuing progesterone also inhibits FSH and LH, so no new follicle develops and no further cycle begins.
After about twelve weeks the placenta takes over progesterone secretion and the corpus luteum is no longer required.
Common mistakes and how to avoid them
The most frequent error is attributing hormones to the wrong source. FSH and LH come from the anterior pituitary; oestrogen comes from the follicle and progesterone from the corpus luteum.
Students often describe the LH surge as negative feedback. It is positive feedback, because high oestrogen stimulates LH rather than inhibiting it.
Another common slip is saying that oestrogen always inhibits the pituitary. Its effect depends on concentration: inhibitory at moderate levels, stimulatory above a threshold.
Many candidates state that the pill works by preventing implantation. Its principal action is preventing ovulation through negative feedback; the endometrial effect is secondary.
In graph questions, answers frequently describe the curves without linking them to events. Each rise and fall should be explained by what is happening in the ovary or uterus.
Finally, candidates often forget that the corpus luteum secretes oestrogen as well as progesterone, which accounts for the second, smaller oestrogen peak in the luteal phase.
Exam technique for "Hormonal control and the menstrual cycle"
For each hormone, be able to state four things: where it is made, what it acts on, what it does, and what controls its secretion. Most questions are answerable from that set.
In graph questions, find the LH spike first and mark it as ovulation at about day fourteen. Every other feature can then be placed relative to it.
When feedback is mentioned, state explicitly whether it is negative or positive and justify the label by saying whether the response reverses or amplifies the change.
For contraception and fertility questions, work from the normal cycle outwards: identify which step is being blocked or stimulated, then explain the consequence.
Use concentrations rather than presence or absence. Oestrogen is present throughout; it is the concentration that determines its effect.
Quick revision summary
FSH from the anterior pituitary stimulates follicle development and oestrogen secretion; LH from the anterior pituitary surges to trigger ovulation and then forms the corpus luteum; oestrogen from the follicle repairs and thickens the endometrium; progesterone from the corpus luteum maintains it. Menstruation occurs when low progesterone allows the endometrium to break down, and the loss of inhibition raises FSH. In the follicular phase, rising oestrogen thickens the endometrium and inhibits FSH by negative feedback so only one follicle matures. Above a threshold concentration, oestrogen switches to positive feedback and stimulates the LH surge, causing ovulation at about day fourteen. In the luteal phase the corpus luteum secretes progesterone, maintaining the endometrium and inhibiting FSH and LH; if no fertilisation occurs it degenerates and the cycle restarts. If implantation occurs, hCG from the embryo maintains the corpus luteum until the placenta takes over. Combined contraceptives maintain high oestrogen and progesterone, inhibiting FSH and LH so ovulation is prevented, and fertility treatment uses FSH or oestrogen-receptor blockers to stimulate follicle development.