Fertilisation, implantation and the hormonal control of pregnancy: HSC Biology Module 5

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.

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.

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.

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.

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.

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.

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.

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.