Inquiry Question 1: How does reproduction ensure the continuity of a species?
Analyse the features of fertilisation, implantation and hormonal control of pregnancy and birth in mammals
A focused answer to the HSC Biology Module 5 dot point on mammalian reproduction. Fertilisation of the egg by sperm to form a zygote, cleavage and blastocyst implantation, the menstrual-cycle context (FSH, LH), hCG maintaining the corpus luteum, progesterone and oestrogen maintaining the endometrium and pregnancy, the placenta, and birth driven by oxytocin positive feedback.
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What this dot point is asking
NESA wants you to analyse the features of three linked processes in mammals: fertilisation (sperm fuses with egg to form a zygote), implantation (the blastocyst embeds in the uterus lining), and the hormonal control of pregnancy and birth. "Analyse" means do more than list facts: show how the parts connect - which hormone maintains which structure, and what type of feedback is at work.
The high-mark answers trace cause and effect through the hormones (FSH and LH set the menstrual-cycle context; hCG maintains the corpus luteum; progesterone and oestrogen maintain the endometrium; oxytocin drives birth) and correctly name negative feedback as the stabiliser of pregnancy but positive feedback as the driver of birth.
The answer
Mammalian reproduction runs as a sequence: a sperm fertilises an egg, the resulting zygote divides (cleavage) into a blastocyst, the blastocyst implants in the uterus, and a cascade of hormones first maintains the pregnancy and finally triggers birth.
Fertilisation
After ovulation, the egg (ovum) travels into the oviduct. Many sperm reach it, but each must first break through the egg's protective coatings. Enzymes released from the sperm head (the acrosome reaction) digest a path so one sperm membrane can fuse with the egg membrane.
The moment one sperm enters, the egg's outer layer changes (the cortical reaction) to block any further sperm - this prevents polyspermy (more than two chromosome sets). The haploid sperm nucleus (n) then fuses with the haploid egg nucleus (n) to form a single diploid zygote (2n). Fertilisation therefore restores the species' diploid number and combines two parents' genes in one new cell.
Cleavage and implantation
The zygote does not grow at first; it undergoes cleavage - rapid mitotic divisions that split it into many small cells while it drifts down the oviduct toward the uterus. It becomes a solid ball (the morula) and then a hollow, fluid-filled blastocyst by about day 5.
The blastocyst's outer layer, the trophoblast, then carries out implantation: it embeds the blastocyst into the endometrium (the thickened, blood-rich lining of the uterus). Implantation anchors the embryo to the maternal blood supply and the trophoblast begins to form the embryonic part of the placenta.
The menstrual-cycle context: FSH and LH
Before pregnancy, the menstrual cycle prepares an egg and the uterus each month. FSH (follicle stimulating hormone) from the anterior pituitary makes a follicle mature; the follicle secretes oestrogen, which thickens the endometrium. A mid-cycle surge of LH (luteinising hormone) triggers ovulation (release of the egg) around day 14. LH then turns the empty follicle into the corpus luteum, which secretes progesterone to maintain the endometrium for a possible pregnancy.
If no fertilisation occurs, the corpus luteum degenerates after about 12 days, progesterone falls, the endometrium is shed (menstruation), and a new cycle begins. Pregnancy must override this monthly reset.
Hormonal control of pregnancy: hCG, progesterone, oestrogen
When the blastocyst implants, its trophoblast secretes human chorionic gonadotrophin (hCG). hCG acts like LH: it maintains the corpus luteum so it keeps producing progesterone and oestrogen instead of degenerating.
- Progesterone maintains the endometrium, suppresses FSH and LH (so no new follicle matures and there is no menstruation), and quietens uterine contractions.
- Oestrogen helps maintain the endometrium, promotes uterine growth and prepares the breasts for milk production.
From about the end of the first trimester, the placenta takes over from the corpus luteum as the main source of progesterone and oestrogen, so the pregnancy continues even once the corpus luteum has degenerated. This whole arrangement is negative feedback: high progesterone suppresses the hormones that would restart the cycle, holding the uterus in a stable, pregnancy-supporting state.
Hormonal control of birth: oxytocin and positive feedback
Near full term, progesterone's calming hold on the uterus weakens (oestrogen rises relative to progesterone) and the uterus becomes sensitive to oxytocin. Birth is then driven by a positive feedback loop:
- The baby's head presses on and stretches the cervix.
- Stretch receptors signal the hypothalamus; the posterior pituitary releases oxytocin.
- Oxytocin makes the uterine muscle (myometrium) contract, pushing the baby harder against the cervix.
- More cervical stretch → more oxytocin → stronger contractions - the loop amplifies itself.
This continues until the baby is delivered. Once born, the cervix is no longer stretched, the stimulus is removed, and oxytocin falls - the loop switches itself off. This is the opposite of the negative feedback that keeps pregnancy stable: positive feedback drives the system to a climax (delivery) rather than back to a set point.
The role of the placenta
The placenta has two jobs. As an exchange organ, it brings maternal and foetal blood close together (without mixing) so oxygen, glucose, amino acids and antibodies pass to the foetus and carbon dioxide and wastes pass to the mother. As an endocrine organ, from the late first trimester it secretes the progesterone and oestrogen that maintain the endometrium and the pregnancy, taking over from the corpus luteum.
In context
Why hCG is the pregnancy-test signal. Home pregnancy tests detect hCG in urine because only an implanted embryo's trophoblast makes it. Its biological job, though, is to keep the corpus luteum alive so progesterone stays high - the test is just a convenient by-product of that signal.
Labour-inducing drugs. Synthetic oxytocin (for example, the drug Syntocinon) is given clinically to start or strengthen labour. It works by feeding into exactly the positive-feedback loop above - more oxytocin drives stronger contractions - which is why its dose is carefully controlled.
Practice questions
Original practice questions graded from foundation to exam level, each with a full worked solution. Try them before revealing the solution.
foundation2 marksDefine fertilisation and state the ploidy of the cell it produces in a mammal with a diploid number of 2n = 64 (the horse).Show worked solution →
1 mark - definition. Fertilisation is the fusion of a haploid male gamete (sperm) with a haploid female gamete (egg/ovum) to form a single cell, the zygote.
1 mark - ploidy and number. The zygote is diploid: the haploid sperm (n = 32) and haploid egg (n = 32) fuse to restore 2n = 64.
One mark for the fusion-of-gametes definition, one mark for stating "diploid" with the correct number (64). A bare "they join" with no mention of haploid gametes fusing does not earn the first mark.
foundation3 marksPlace these events of early human development in the correct order and name the structure that implants: cleavage, fertilisation in the oviduct, implantation in the endometrium, formation of the blastocyst.Show worked solution →
- 1 mark - correct sequence
- Fertilisation in the oviduct → cleavage → formation of the blastocyst → implantation in the endometrium.
- 1 mark - what cleavage is
- Cleavage is a series of rapid mitotic divisions of the zygote (with little overall growth) as it travels down the oviduct, producing a solid ball of cells (the morula) and then a fluid-filled blastocyst.
- 1 mark - the implanting structure
- The blastocyst implants; specifically its outer trophoblast layer embeds in the endometrium of the uterus.
One mark for the order, one for describing cleavage as mitotic division, one for naming the blastocyst (not "the zygote" or "the embryo") as the structure that implants.
foundation2 marksName the hormone secreted by the early embryo after implantation and state its single most important role in early pregnancy.Show worked solution →
1 mark - the hormone. Human chorionic gonadotrophin (hCG), secreted by the trophoblast/chorion of the implanted embryo.
1 mark - its role. hCG maintains the corpus luteum, so the corpus luteum keeps secreting progesterone (and oestrogen), which maintains the endometrium and prevents menstruation in early pregnancy.
One mark for naming hCG, one mark for the "maintains the corpus luteum so progesterone continues" link. Saying only "it shows up on a pregnancy test" does not earn the role mark.
core4 marksExplain how the corpus luteum and the hormone hCG work together to maintain the endometrium in the first weeks of pregnancy, and explain what would happen to the pregnancy if hCG were not produced.Show worked solution →
- Corpus luteum secretes progesterone (1 mark)
- After ovulation the empty follicle becomes the corpus luteum, which secretes progesterone (and some oestrogen). Progesterone maintains the thickened, vascular endometrium ready to support an embryo.
- hCG rescues the corpus luteum (1 mark)
- Without pregnancy the corpus luteum normally degenerates after about 12 days, progesterone falls and menstruation occurs. When implantation succeeds, the trophoblast secretes hCG, which acts like LH to keep the corpus luteum alive and secreting progesterone.
- Result - the endometrium is maintained (1 mark)
- Sustained progesterone keeps the endometrium intact and inhibits the FSH/LH that would start a new cycle, so menstruation does not occur and the embryo is not lost.
- Consequence of no hCG (1 mark)
- Without hCG the corpus luteum would degenerate on schedule, progesterone would fall, the endometrium would break down (menstruate), and the embryo would be shed - the pregnancy would fail.
Full marks need the cause-and-effect chain (hCG → corpus luteum maintained → progesterone sustained → endometrium maintained) AND the counterfactual (no hCG → progesterone falls → endometrium breaks down). Naming the hormones without the causal links caps the answer at 2 marks.
core4 marksDescribe the role of the placenta in maintaining a pregnancy in mammals, referring to BOTH its exchange function and its endocrine (hormonal) function.Show worked solution →
- Exchange function (1 mark)
- The placenta is the organ where maternal and foetal blood come into close contact without mixing. Oxygen, glucose, amino acids and antibodies diffuse from mother to foetus, and carbon dioxide and other wastes pass from foetus to mother.
- Why this matters (1 mark)
- This exchange supplies the growing foetus with everything it needs and removes its wastes, allowing development inside the uterus until birth.
- Endocrine function (1 mark)
- From roughly the end of the first trimester the placenta takes over from the corpus luteum as the main source of progesterone and oestrogen.
- Why the endocrine role matters (1 mark)
- Placental progesterone and oestrogen maintain the endometrium, support foetal growth and prepare the breasts for lactation, so the pregnancy continues even after the corpus luteum degenerates.
Markers award one mark for each function described AND one for its significance. A response that only mentions "oxygen and food pass across" misses the endocrine half and is capped at 2 marks.
core3 marksDistinguish between the negative feedback that controls progesterone during pregnancy and the positive feedback that controls oxytocin during birth.Show worked solution →
- Negative feedback during pregnancy (1 mark)
- High progesterone (from the corpus luteum, then placenta) inhibits the hypothalamus and pituitary, suppressing FSH and LH. This stops new follicles maturing and ovulation, keeping the system stable - the response opposes further change, which is negative feedback.
- Positive feedback during birth (1 mark)
- Stretching of the cervix triggers oxytocin release; oxytocin causes uterine contractions; contractions push the baby's head harder against the cervix, stretching it more, which causes still more oxytocin - the response amplifies the stimulus, which is positive feedback.
- The key distinction (1 mark)
- Negative feedback returns the system toward a set point and keeps pregnancy stable; positive feedback drives the system away from a set point to a climax (delivery), then stops once the baby is born and the cervix is no longer stretched.
One mark for correctly describing each loop and one mark for the explicit "opposes change vs amplifies change" distinction. Labelling oxytocin as negative feedback is the classic error and forfeits two marks.
exam6 marksAnalyse how the hormonal control of pregnancy changes the role of negative feedback that normally governs the menstrual cycle, from ovulation through to the maintenance of the endometrium. In your answer refer to FSH, LH, oestrogen, progesterone and hCG.Show worked solution →
- Baseline menstrual cycle (1 mark)
- In a normal cycle, FSH from the anterior pituitary stimulates a follicle to mature and secrete oestrogen. Rising oestrogen first inhibits then, at a threshold, triggers an LH surge (a switch to positive feedback) that causes ovulation around day 14.
- Corpus luteum and progesterone (1 mark)
- After ovulation, LH converts the empty follicle into the corpus luteum, which secretes progesterone (and oestrogen). Progesterone maintains the endometrium and, by negative feedback, inhibits FSH and LH so no new follicle develops.
- No pregnancy - the cycle resets (1 mark)
- If fertilisation does not occur, the corpus luteum degenerates after about 12 days, progesterone and oestrogen fall, the endometrium is shed (menstruation), and the drop in negative feedback lets FSH rise to begin a new cycle.
- Pregnancy overrides the reset - hCG (1 mark)
- If implantation occurs, the trophoblast secretes hCG. hCG mimics LH and maintains the corpus luteum past its normal lifespan, so progesterone stays high instead of falling.
- Sustained negative feedback maintains pregnancy (1 mark)
- Continued high progesterone and oestrogen maintain the endometrium and keep FSH and LH suppressed, so there is no new follicle, no ovulation and no menstruation for the duration of the pregnancy.
- Analysis - the shift (1 mark)
- The same negative-feedback machinery is repurposed: in the cycle it resets monthly because progesterone falls, whereas in pregnancy hCG (then the placenta) keeps progesterone high so the feedback that would normally restart the cycle instead locks it down, holding the uterus in a pregnancy-supporting state.
This is a Band 5/6 response. Full marks require the logically sequenced chain through all named hormones AND an analytical statement that the negative-feedback loop is sustained (not abolished) by hCG/placental progesterone. Listing the hormones without explaining how hCG changes the outcome caps the answer in the middle band.
exam7 marksA pregnant mammal goes into labour. Explain the hormonal mechanism that drives birth, identifying the type of feedback involved, and assess why this mechanism is well suited to the process of birth but would be dangerous if used to control the menstrual cycle.Show worked solution →
- Trigger (1 mark)
- Near full term, falling progesterone (relative to oestrogen) and pressure from the mature foetus increase the sensitivity and stretch of the uterus and cervix, priming the system for labour.
- Oxytocin release (1 mark)
- Stretch of the cervix is detected by receptors that signal the hypothalamus; the posterior pituitary releases oxytocin into the blood.
- Contractions (1 mark)
- Oxytocin stimulates the smooth muscle of the uterus (myometrium) to contract, pushing the foetus down against the cervix.
- The positive-feedback loop (1 mark)
- Greater pressure on the cervix stretches it further, which triggers more oxytocin, which causes stronger contractions - an amplifying loop. This is positive feedback: the response intensifies the original stimulus rather than opposing it.
- Termination (1 mark)
- The loop runs to a climax - delivery of the baby. Once the baby is born the cervix is no longer stretched, the stimulus is removed, oxytocin falls and the loop switches off.
- Assess - suited to birth (1 mark)
- Positive feedback is ideal here because birth needs a rapid, self-amplifying, one-off escalation to completion; a negative-feedback loop would damp the contractions and stall the delivery.
- Assess - dangerous for the cycle (1 mark)
- The menstrual cycle must be stable and repeating, returning to a set point each month; a positive-feedback loop has no stable set point and would spiral out of control, so the cycle relies on negative feedback (progesterone suppressing FSH/LH) instead. The judgement is that the feedback type must match the goal: amplify-to-climax for birth, stabilise-and-repeat for the cycle.
Top-band response. Full marks require the complete oxytocin loop, the explicit "positive feedback" label, the self-terminating step, AND a genuine assessment weighing why positive feedback suits a one-off climax but is unsuitable for a stable repeating cycle. Describing the loop without the assessment caps the answer around 4-5 marks.